Because onychomycosis requires long-term systemic therapy, it is essential to diagnose the infection accurately. Although in theory this diagnosis should be simple, results achieved in practice are uneven. Clinicians should be aware of the need to collect an adequate specimen and of the possible need for repeat collections. Direct microscopic techniques for examination of nail scrapings include 10% potassium hydroxide (KOH) with Parker ink, 10% KOH with dimethyl sulfoxide, and 10% KOH with Calcofluor white. When interpreting fungal culture results, it should be noted that negative results are frequent and that contaminant yeasts and molds are common, but that nondermatophytes such as Candida , Scopulariopsis , and Scytalidium can infrequently cause onychomycosis. (J Am Acad Dermatol 1999;40:S3-8.)