Background: Onychomycosis is a common nail infection, often resulting in nail plate damage and deformity. Topical lacquer treatments have negligible efficacy. Oral treatments, although more efficacious, are limited by drug interactions and potential hepatotoxicity. Objective: We investigated the safety and efficacy of efinaconazole 10% solution (efinaconazole), the first triazole antifungal developed for distal lateral subungual onychomycosis. Methods: Two identical, multicenter, randomized, double-blind, vehicle-controlled studies were conducted in patients with toenail distal lateral subungual onychomycosis (20%-50% clinical involvement [study 1: N = 870, study 2: N = 785]). Patients were randomized (3:1) to efinaconazole or vehicle, once daily for 48 weeks, with 4-week posttreatment follow-up. Debridement was not performed. The primary end point was complete cure rate (0% clinical involvement of target toenail, and both negative potassium hydroxide examination and fungal culture) at week 52. Results: Mycologic cure rates were significantly greater with efinaconazole (study 1: 55.2%, study 2: 53.4%) compared with vehicle (P < .001). The primary end point, complete cure, was also significantly greater for efinaconazole (study 1: 17.8% vs 3.3%, study 2: 15.2% vs 5.5%, P < .001). Treatment success (percent affected target toenail [0%-≤10%]) for efinaconazole ranged from 21.3% to 44.8% in study 1 and from 17.9% to 40.2% in study 2, compared with 5.6% to 16.8% and 7.0% to 15.4%, respectively, with vehicle. Adverse events associated with efinaconazole were local site reactions (2%) and clinically similar to vehicle. Limitations: A period of 52 weeks may be too brief to evaluate a clinical cure in onychomycosis. Conclusions: Once daily topical efinaconazole appears to be a viable alternative to oral treatment options for onychomycosis. © 2012 by the American Academy of Dermatology, Inc.