BACKGROUND: Onychomycosis is a common disease that remains a difficult disorder to treat despite the introduction of new topical agents; and not all patients are cured. Clinical experience leads us to suggest a number of host-related factors can affect the chance of cure, but studies supporting these observations are currently lacking. Although many studies, particularly on topical agents, rely on severity classification when selecting patients for inclusion, a pilot study was unable to demonstrate any prognostic value of the extension of nail involvement. In addition, no universal severity classification exists, and most studies do not report prognostic factors. OBJECTIVE: To investigate the efficacy of efinaconazole topical solution, 10% in patients with mild-to-moderate onychomycosis and determine the impact of baseline severity on treatment outcome. METHODS: Post hoc pooled analysis of two identical, multicenter, randomized, double-blind, vehicle-controlled studies in 1655 patients aged 18-70 years with a clinical and mycological diagnosis of mild-to-moderate dermatophyte toenail onychomycosis (20-50% clinical involvement). Patients were randomized (3:1) to efinaconazole 10% solution or vehicle, once-daily for 48 weeks, with 4-week post treatment follow-up. Efficacy criteria included clear nail (0% target nail plate involvement), almost clear nail (≤5% target nail plate involvement), and clinical treatment success (≤10% target nail plate involvement) at week 52. For the post hoc analysis, patients were classified as mild (20%-29% nail involvement), moderate (30%-39%), and moderately severe (40%-50%) at baseline. RESULTS: Overall, 25%, 23%, and 52% of patients had mild, moderate, or moderately severe disease at baseline. Baseline nail involvement did not appear to predict treatment outcomes. The proportion of patients with mild disease who had a clear nail progressively reduced by week 36 (58%) and week 48 (41%), and even further by week 52 (37%). Of the 237 patients treated with efinaconazole who were 'clear' at week 52, 37%, 24%, and 39% had mild, moderate or moderately severe disease respectively at baseline. The majority of patients (N=634) saw at least a 50% improvement in their target toenail by week 52. Almost half of these patients (N=312, 49.2%) were moderately severe at baseline. CONCLUSIONS: This post hoc analysis supports previous data showing good efficacy of efinaconazole in mild onychomycosis. The relative contribution to overall efficacy results at week 52 of patients with moderate or moderately severe disease was unexpected for a topical therapy, and warrants further study, especially as they represent the majority of patients enrolled in the two studies. It is possible that comparable efficacy can be achieved in these more severe patients with longer treatment courses, or follow-up. J Drugs Dermatol. 2018;17(2):175-178.