Four open, prospective, postmarketing studies with consistent protocols were undertaken, involving over 25,000 patients and nearly 3600 physicians in four European countries (UK, Netherlands, Germany and Austria). The purpose was to determine the tolerability of terbinafine when prescribed to a large, unselected patient population in general medical practice. Patients were recruited from dermatology departments and from general and family practitioners. All received at least one dose of terbinafine. No specific exclusion criteria were applied. The only treatment instructions provided were those contained in the manufacturer's product information. Patients were monitored for adverse events at baseline, during treatment, and at the end of treatment. Of the 25,884 patients entering the study, 38.6% (9991) had concomitant disease, 42.8% (11,078) were taking other medications, and 22.7% (5876) were older than 60 years. The predominant indication for prescribing terbinafine was onychomycosis. Mean duration of treatment was 13.2 weeks. No adverse events were reported in 89.5% (23,167) of patients. The remaining 10.5% (2717) reported one or more adverse events, primarily gastrointestinal (4.9%) or dermatological (2.3%). A total of 115 serious adverse events were recorded, but of these only four were probably and eight possibly related to terbinafine. There were no reported drug-drug interactions, even in patients receiving concomitant medications metabolized by cytochrome P-450 enzymes. There were also no clinically significant drug-disease interactions. In conclusion, terbinafine is well tolerated in an unselected general practice population. This confirms its good tolerability previously established during controlled clinical trials.