Onychomycosis is a very common disease, especially in podiatric medical practice. It can be associated with significant patient distress, major disability and pain, and is challenging to treat successfully. This is a case study of a 41-year-old man with distal lateral subungual onychomycosis of 5 years' duration. Forty percent of the great toenail was affected and a total of six toenails were involved. Baseline fungal cultures were positive for Trichophyton rubrum. This patient was treated with efinaconazole 10% solution, a new topical antifungal, once daily for 48 weeks. Mycological cure was noted at the first assessment period (12 weeks), and compete cure was seen at follow-up. This case study alerts physicians to a promising new topical treatment for onychomycosis under development, and to the importance of mycological cure as an early indicator of treatment success.