We measured the prevalence of pedal onychomycosis in patients with psoriasis attending dermatologists' offices (4 in Canada, 2 in USA).The prevalence of pedal onychomycosis in psoriatics (n = 561) was 13%. The odds of psoriasis patients having onychomycosis was 56% greater than non-psoriatics of the same age and sex (p = 0.02). In a psoriatic with abnormal-appearing nails consideration should be given to excluding onychomycosis and effectively treating it, prior to managing the psoriatic nails with therapies, such as topical or intralesional corticosteroids.