There is some controversy about the prevalence of onychomycosis in patients with psoriasis compared to non-psoriatics. We therefore measured the prevalence of toenail onychomycosis in psoriatics and non-psoriatics attending dermatologists' offices. None of the patients had a referring diagnosis of onychomycosis. The prevalence of pedal onychomycosis in psoriatics (n = 561) was 13%. The odds of patients with psoriasis having onychomycosis was 56% greater than non-psoriatics of the same age and sex (P = 0.02). In the psoriatics, when the toenails were clinically abnormal, the prevalence of onychomycosis was 27%. The odds of developing onychomycosis increased with age (P < 0.0001) and the odds of men developing onychomycosis was 2.5 times that of women (P = 0.0001). The duration of psoriasis did not significantly affect the odds of developing onychomycosis. The fungal organisms recovered from psoriasis subjects with onychomycosis were similar to those in the normal population with onychomycosis (P = 0.58).