We investigated visitors to a swimming pool in Reykjavik to determine whether onychomycosis of the toenails is more prevalent in swimmers than in the general population, where the prevalence is believed to be between 3 and 8%. A total of 266 swimmers over the age of 17 years were interviewed and examined. When an onychomycosis was suspected a nail specimen was taken for mycological examination. Onychomycosis was clinically suspected in 105 cases (40%). In 60 cases (23%) a dermatophyte infection was confirmed by culture and 14 cases (5%) were microscopy-positive only. The prevalence of culture-positive onychomycosis was 15% in women and 26% in men. Our results suggest that onychomycosis of the toenails is at least 3 times more prevalent in swimmers than in the rest of the population.