Terbinafine (Lamisil) has been registered throughout the world for the treatment of finger and toenail onychomycosis. The recommended duration of treatment of toenail onychomycosis based on phase III studies is 12 weeks. This study was designed to determine: (i) if patients in whom the proximal part of the toenails was not affected respond as well after 6 weeks treatment as after 12 weeks treatment; (ii) to identify factors which may allow selection of patients for shorter treatment duration; and (iii) confirm that 6 weeks therapy is sufficient in fingernail mycosis. One hundred and forty-eight patients received 250 mg terbinafine daily for either 6 or 12 weeks in a double-blinded manner, and were allowed until 48 weeks after start of therapy. Cure of the nail infection was defined as negative mycological tests (mycological cure) and progressive growth of normal nail (clinical cure). Mycological cure was recorded in 43 of 72 (59.7%) in the 6-week group and 55 of 76 (72.4%) in the 12-week group. In those who completed the study per protocol in the 6-week group, 34 of 61 (55.7%) were cured mycologically corresponding to 46 of 56 (82.1%) in the 12-week group. The overall clinical and mycological cure rates for the two groups were 28 of 61 (45.9%) and 33 of 56 (58.9%), respectively. In the small number of patients with associated fingernail infection, all were improved and six of eight (75.0%) were cured after a duration of treatment of 6 weeks. A priori risk factors for failure of cure could not be identified in either group. However, shorter duration of disease prior to treatment and no involvement of the big toenail was associated with a trend toward better responses in both groups. It can be concluded from this study that, in toenail mycosis without visible matrix involvement, 6 weeks treatment of terbinafine is generally not sufficient, whereas fingernail infections respond well to this short therapy.