The route of terbinafine (Lamisil) penetration into the stratum corneum after oral administration was studied in five patients. Serial skin surface biopsy samples of stratum corneum were taken after 1, 3, 7 and 14 days of treatment and terbinafine levels measured by high-performance liquid chromatography. Terbinafine was detected in the stratum corneum as early as 24 hours after the start of treatment in three of five patients. In all cases terbinafine was first detected in the deeper levels of stratum corneum, indicating that the route of delivery to stratum corneum after oral administration is initially by epidermal diffusion.