New antifungals for oral therapy have improved the chances of healing onychomycoses. Nevertheless, in daily practice the failure rate is 20-30% both with itraconazole and terbinafine. In our investigations for the fungicidal effects of terbinafine, we could show that the same strain of Trichophyton rubrum or T. mentagrophytes in the rest period needs 1000 times higher concentration of terbinafine (2.0 to 0.002 microgram/ml) for complete fungal killing as in the growth phase. Thus resting fungi in the nail are not harmed, and if the concentration of terbinafine is lower than the MIC for the fungi, they can cause a relapse. Itraconazole is a fungistatic agent. Its concentration in the nail plate must be higher than the MIC for the causative fungi for a long time-10 to 12 months. The usual short-term treatment of onychomycosis over 3 months can be insufficient in individual cases such as patients with disorders, e.g. circulatory disorders or slowed nail growth.