Until now, there has been no agreement on criteria defining resolution of onychomycosis. Most published reports use clinical and mycological cure, which comprises a completely normal-appearing nail plate, and negative nail culture and microscopy results, as the end point for defining success of therapeutic intervention. Reported here is the definition of onychomycosis, which delineates both primary and secondary criteria for diagnosis of onychomycosis and identifies clinical and laboratory parameters to define a resolved fungal nail infection. Onychomycosis cure is defined by the absence of clinical signs or the presence of negative nail culture and/or microscopy results with one or more of the following minor clinical signs: (1) minimal distal subungual hyperkeratosis; and (2) nail-plate thickening. Clinical signs indicative of persistent onychomycosis at the end of the observation period include (1) white/yellow or orange/brown streaks or patches in or beneath the nail plate; and (2) lateral onycholysis with subungual debris. Although nail appearance will usually continue to improve after cessation of therapy, the nails may have a persistent abnormal appearance even in cases where treatment has been effective. © 2007 American Academy of Dermatology, Inc.