Onychomycosis is difficult to cure as this requires eradication of the primary infection and protection of new areas of growth from reinfection. A new topical treatment (K101) has been developed. The aim of this study was to assess the efficacy, safety and tolerability of K101 treatment of distal subungual onychomycosis. This was a 24-week (plus 2-week washout), multicentre, randomised, double-blind, placebo-controlled study in 493 patients with distal subungual onychomycosis (K101, n = 346; placebo, n = 147), stratified according to degree of nail involvement. More patients with ≤50% nail involvement achieved the primary endpoint (mycological cure after 26 weeks) in the K101 group (27.2%) than placebo (10.4%; P = 0.0012). Proportions for patients with 51–75% involvement were 19.1% for K101 and 7.0% for placebo (not significant). More patients applying K101 than placebo judged that their condition had improved from week 2 (P = 0.0148) to week 24 (P = 0.0004). No safety issues were identified. K101 provides early visible improvements in nail appearance and a clinically meaningful antifungal activity.