The efficacy and safety of the butterbur leaf extract Ze 339 (carbon dioxide extract from the leaves of Petasites hybridus L., 8 mg petasines per tablet) were tested in patients with seasonal allergic rhinitis. In an open postmarketing surveillance study, 580 patients were treated with an average of 2 tablets of Ze 339 daily for 2 weeks. Symptoms of rhinorrhea, sneezing, nasal congestion, itchy eyes and nose, red eyes, and skin irritation were evaluated on a visual analogue scale. Symptoms of seasonal allergic rhinitis improved in 90% of patients. Differences observed before and after therapy were significant and clinically relevant for all symptoms. Improvement reported by the end of the study was found to be inversely related to symptom severity as described at baseline. Efficacy, tolerability, and improvement in quality of life were positively rated by 80%, 92%, and 80% of patients, respectively. A total of 44% of patients were given an antiallergic comedication. This combination did not result in a better effect than was attained with Ze 339 monotherapy. Adverse events occurred at a rate of 3.8%, and gastrointestinal complaints were predominantly nonspecific. Results of this postmarketing surveillance trial are consistent with observations documented in previous randomized, double-blind, prospective, controlled trials of the same extract that were conducted according to Good Clinical Practice (GCP). Butterbur leaf special extract Ze 339 was confirmed by 3 GCP trials and 2 postmarketing surveillance trials to be safe and efficacious in the treatment of patients with seasonal allergic rhinitis.