Onychomycosis is a persistent fungal infection of the toenails or fingernails that is usually not painful but is unsightly and can affect a patient's quality of life by interfering with footwear. It may affect up to 30% of the population by age 60. In more that 99% of cases, it is caused by dermatophytes, the most common of which are Trichophyton rubrum and Trichophyton mentagrophytes. Each of the four clinical types of onychomycosis, as defined by the route of fungal invasion, has a characteristic appearance, but other diseases, particularly psoriasis, may have a similar appearance. Proper management, therefore, includes confirmation of fungal infection by potassium hydroxide slide preparation and culture. Traditionally, pharmacologic treatment has been less than optimal. In many cases, griseofulvin, the first oral agent approved for onychomycosis in the United States, must be given for 1 year or more to be effective. Low cure rates are related to poor bioavailability and the fungistatic rather than fungicidal effect of the drug. Newer agents, such as oral itraconazole and oral terbinafine, promise to substantially increase cure rates while shortening treatment duration. Oral terbinafine is potently fungicidal against dermatophytes and has proven efficacious with regimens as brief as 12 weeks when the nail is not 100% involved.