Onychomycosis among diabetic patients has been reported in some studies to be of high prevalence. This study aimed to investigate the prevalence of onychomycosis among diabetic patients at a Danish University Hospital. Clinical and mycological examinations were performed on type 1 and 2 diabetic patients from in- and out-patient clinics. A total of 271 patients were enrolled, 72% males, mean age 61.3 years, 26% of the patients had diabetes type 1. The prevalence of toe nail onychomycosis (positive culture and/or microscopy) was 22% (n = 59) of which 55 cases were caused by dermatophytes (93%) and 4 cases by yeasts (7%). A correlation was found between onychomycosis and age (p =0.02) and severity of nail changes (p <0.001), respectively. However, no significant correlation was found to gender, type of diabetes, lower extremity arterial disease, neuropathy, toe amputation or oedema. Onychomycosis occurred with a high prevalence in diabetic patients, especially among older patients and those with severe nail changes.