Successful treatment of onychomycosis in the infection site depends not only on achieving the minimal inhibitory concentration (MIC) of the antifungal agent, usually determined on fresh, proliferating fungal strains, but also on the effectivity against fungal spores dormant in nail keratin. Ciclopiroxolamine and terbinafine were investigated for their fungicidal properties against proliferating and dormant dermatophyte strains. While ciclopiroxolamine was 100% effective against Trichophyton mentagrophytes (50 microg ml(-1)) and Microsporum canis (5 microg ml(-1)) both in the proliferative and dormant phase after 5 days of incubation, the same result was achieved under identical test conditions with 0.002 microg ml(-1) terbinafine using T. mentagrophytes as test organism in the proliferative and 2.0 microg ml-1 in the dormant phase. The terbinafine concentrations of 0.52 microg g(-1) measured in the nail are well below 2.0. This explains the high treatment failure and relapse rates observed under monotherapy of toenail onychomycosis even with modern antifungals. Consequently, combined therapy is recommended, beginning with atraumatic removal of the affected toenails and continuing with an antifungal nail lacquer combined with a systemic antifungal.