Onychomycosis is a nail infection caused by dermatophytes, Candida and molds. We aimed to obtain an estimated frequency of onychomycosis in out-patients in private practice through a survey in ten representative cities of Mexico. 12,637 ambulatory patients voluntarily agreed to participate in this National Survey, answering 17 questions on onychomycosis, regardless if they had or did not have any clinical suspicion of onychomycosis. 53% of them were seen for the first time and their main complaint was not onychomycosis. The study was performed in the private offices of 300 physicians in different cities. 48% were clinically diagnosed with onychomycosis. Toenails were affected in 88%, fingernails in 5% and both in 7%. Onychomycosis was diagnosed more frequently in the nails of the first toes and of the thumbs. The main complaint was nail thickening and aesthetic changes. Other associated diseases were diabetes (22%) and arterial hypertension (21%). This survey showed the high frequency of onychomycosis. An intentional search with mycological confirmation is needed in out-patients attending general practice with other complaints, to detect undiagnosed cases.