Terbinafine (Lamisil) is an antifungal drug, belonging to a class of drugs called the allylamines, which has recently become available for clinical use. This is a report of four special features which emerged during evaluation of the systemic use of orally administered terbinafine in the treatment of 2500 patients during the last 5 years. These features include: (i) distribution of terbinafine in skin, hair and nail tissue; (ii) use in short-duration treatment of chronic tinea pedis and onychomycosis; (iii) use in patients with serious infections often associated with local or generalized immunological defects; and (iv) tolerability and safety of the drug. The diffusion of terbinafine into thickened, chronically infected tissue and into nails, and its recognized fungicidal action are the most likely features responsible for its success in the treatment of chronic fungal diseases, including those with immunological defects. Terbinafine is well tolerated, particularly when compared with other available systemic antifungal drugs.