Urticaria. Close-up of urticaria on the limb of an 18-year-old female patient. Urticaria, also known as nettle rash or hives, is a skin condition in which a rash of itchy wheals or lumps develops on the skin, usually on the trunk or limbs. The wheals appear in a matter of minutes and usually fade after a few hours. Urticaria most often occurs as an acute allergic response, triggered, for instance, by particular foods or drugs. It can also result from heat, cold, sunlight or, less frequently, other medical conditions like cancer.