Although acute urticaria is common, its eliciting factors, clinical course and therapeutic responsiveness have not been intensively investigated. We have therefore prospectively studied all patients with acute urticaria attending the department of dermatology (n = 72) and a rural dermatology office (n = 37) during the course of 1 year. After a standardized history and physical examination, patients were randomized into treatment with either loratadine (10 mg/day for 3 days) or prednisolene (50 mg/day for 3 days). All patients were followed up until complete remission. Most patients suffered from moderate (42%) to severe (40%) disease. Possible eliciting factors were identified in less than 50% of the cases. Associated upper respiratory tract infections were found most commonly (39.5%), followed by possibly eliciting drugs, mostly analgesics (9.2%) and suspected food intolerance (0.9%). The course of the disease was self-limited in all cases, the longest episode lasting for 3 weeks. Both treatment regimens were effective in controlling whealing, but in corticosteroid-treated patients, symptoms ceased earlier, with complete remission occurring within 3 days of treatment in 93.8%, compared to 65.9% of patients treated with loratadine (p < 0.001). Acute urticaria is thus frequently idiopathic and only rarely associated with IgE-mediated events. It is, however, largely self-limited, with prompt response to symptomatic treatment.