You probably are familiar with hives-red, swollen and very itchy welts that form on the skin. The scientific name for hives is urticaria. About 20 percent of the population develop them at some time in their lives. Hives often appear suddenly, and last only a few hours-or seldom more than six weeks. Each hive lasts no more than 24 hours, and often goes away in 2 to 4 hours.

For some people, though, the hives don't go away, or they go away then recur frequently. This may go on for months or, rarely, for years. This is called chronic urticaria. The condition is not life-threatening. For people who have it, though, it can cause distress and disability.

Chronic urticaria occurs in both sexes but seems to be somewhat more common in middle-aged women. About 30 percent of adults with chronic urticaria also have angioedema. Areas of widely spread swelling occur, and most often affect the lips or eyelids.

Hives are an allergic-like reaction of the skin to a usually harmless substance. Acute cases of hives can be triggered by any number of substances. Common causes are medications, foods, insect bites or stings.

The cause of chronic urticaria is often hard to pinpoint. Although the cause is currently unknown, it is thought to be related to a person's own immune system triggering the reaction.

Some types of chronic urticaria worsen with increased blood flow as a result of heat, exertion, emotional stress, alcoholic drinks, fever or hyperthyroidism. Increases in hives are fairly common during premenstrual periods as well. Some people get pressure urticaria on certain parts of the body as a result of the work they do. People who sit all day can get hives on their buttocks. When people wear their belts too tight, they can get pressure urticaria around the waist.

People with cholinergic urticaria get hives with heat, exercise or emotional stress. Small welts appear within a larger area of redness, often on the neck or upper chest.

For those who get angioedema, it usually affects only the lips or eyelids. In rare cases, though, it can affect the throat. People who have trouble swallowing or breathing should seek emergency care promptly.

Diagnosis begins with a careful medical history and physical exam. Your doctor will check first to make sure chronic hives are not a symptom of a more serious illness such as hepatitis or hyperthyroidism.

A few patients with chronic hives have urticarial vasculitis. Small blood vessels in the skin are inflamed. The hives don't itch quite as much, but they last longer, the area stays red after the hives are gone, and the patient may have aching joints or fever. These symptoms may suggest systemic lupus erythematosus or possibly cancer, but these associations are extremely rare.

Once other illnesses are ruled out, the doctor will ask questions about contact with substances that could have caused an allergic reaction. Some questions will be about your activities. About 15 to 20 percent of chronic causes are physical, triggered by cold, heat, light or exercise.

If the medical history does not suggest what is causing the hives, other measures are taken:

• The patient is asked to keep a diary for a week or two, recording all activities, everything ingested, and when and where hives occur. This may suggest the cause.
• The doctor may order lab tests. If urticarial vasculitis is suspected, a skin biopsy might be ordered.
• Sometimes the patient is put on an allergen elimination diet. The diet is free of the foods that are common causes of allergic reactions. Food reactivity is confirmed if symptoms go away and reoccur once the food is reintroduced. The specific food causing the problem is pinpointed by modifying the patient's diet. Often, though, these measures do not identify the specific cause of chronic hives. In these cases the disease is called "idiopathic." Experts now think the cause may be antibodies in the blood that act against the body's own tissues.

Avoid the cause, if it is known, to prevent the reaction. When the cause is not known, you must simply wait for the condition to go away over time.

If angioedema is a problem, limit exposure to heat, exertion, emotional stress, alcoholic drinks and nonsteroidal antiinflammatory pain relievers such as aspirin. These can aggravate the allergic response.

Take medications to control the itching. They provide excellent relief in most patients:

• Most people now use nonsedating antihistamines: astemizole (Hismanal), loratadine (Claritin), fexofenadine (Allegra) and cetirizine (Zyrtec). They block what are called H1 receptors in tissue. If these medications don't help, the antihistamines cimetidine or ranitidine may be added; they block H2 receptors. The combination of the two types of blockers sometimes provides better relief. Treatment must be individualized and monitored under close medical supervision.
• For more severe urticaria, the more potent antihistamines, hydroxyzine and doxepin, may be used. They do cause drowsiness and other side effects, but the side effects lessen with continued use.
• Corticosteroid medications such as prednisone will control urticaria. They are seldom used because of serious side effects with prolonged use. Sometimes they are needed to treat pressure urticaria or urticarial vasculitis. Doctors prescribe the lowest possible effective dose for the shortest period of time.
• Epinephrine injections may be given for people with angioedema, especially when the throat is swollen.

Much research on urticaria is underway. This will lead to even better treatment in the future. Several new drugs are awaiting approval for use in the United States to treat hives. They act on different skin cells or block mediators other than histamine and will offer another choice for people who get poor relief from current products.