Onychomycosis is common in adults and constitutes 20% of all nail disease. Widely used methods to confirm a clinical diagnosis of onychomycosis often yield negative results (ie, potassium hydroxide [KOH] preparation) or are slow (ie, dermatophyte cultures). We report a case of onychomycosis in which we used in vivo confocal microscopy to diagnose the disease; we also correlated our findings with results from routine KOH preparations. On the basis of our findings, we hypothesize that in vivo confocal microscopy may be faster and more accurate than the conventional microscope used in KOH preparations in the diagnosis of onychomycosis. (J Am Acad Dermatol 2000;42:214-6.)