OBJECTIVE: To examine possible risk factors related to onychomycosis. BACKGROUND: Onychomycosis is a common disease with multifactorial aetiology, but little is known about the risk factors for this disease. PATIENTS AND METHODS: Questions related to signs, symptoms and possible risk factors associated with onychomycosis were sent to 3992 persons aged 16 years and older selected randomly from the Icelandic National Registry. Patients with suspected onychomycosis, based on photographs, were offered mycological examination. Data from the questionnaire and the results of mycological examination were used to calculate the odds ratio (OR) for several factors that might be associated with onychomycosis. RESULTS: Two thousand four hundred and eighty-six subjects responded to the questionnaire. Prevalence for mycologically determined onychomycosis was 11.1% in the Icelandic population. A history of the following factors more than doubled the risk of onychomycosis: cancer (OR 3.44; 95% CI 1.15-10.35), psoriasis (OR 2.44; 95% CI 1.61-3.72), tinea pedis interdigitalis (OR 3.93; 95% CI 3.11-4.95), the moccasin form of tinea pedis (OR 4.26; 94% CI 3.34-5.45), parents with onychomycosis (OR 2.59; 95% CI 1.89-3.53), children with onychomycosis (OR 3.48; 95% CI 2.05-5.88), spouse with onychomycosis (OR 2.53; 95% CI 1.72-3.72), regular swimming activity (OR 2.57; 95% CI 2.00-3.30) and age 50 years or older (OR 2.74; 95% CI 2.19-3.42). CONCLUSIONS: Several risk factors are associated with onychomycosis. Knowledge of these risk factors is important when treating and educating patients with onychomycosis.