Onychomycosis is a fungal infection of the nails which, although unsightly in appearance, is often considered to be a cosmetic problem. This research reports on the development and performance of a quality-of-life instrument to measure the impact of this disease on the patients' mental and general health, social functioning, pain, and self-confidence. In telephone interviews, 680 members of a health maintenance organization (299 with onychomycosis and 381 without) were asked a battery of items regarding quality of life (mental and social functioning, self-esteem, pain) and specific problems and symptoms related with one's nails. Persons with onychomycosis had significantly poorer ratings compared with the healthy persons with respect to general health (p = 0.02) and bodily pain p < 0.001). Persons with onychomycosis also had significantly (p < 0.05) poorer ratings for mental health, social functioning, health concern, physical appearance, and functional limitations associated with activities involving standing on one's feet or working with one's fingers. This study is the first to document the impact of onychomycosis on an individual's quality of life. Persons with onychomycosis may adapt to this condition, but they continue to experience embarrassment and discomfort that reduces their quality of life.