This study was carried out to determine whether slow nail growth is a predisposing factor for onychomycosis or if onychomycosis results in slow nail growth. Forty-nine patients with unilateral onychomycosis of the great toenail were enrolled and classified in two groups according to the size of affected area, i.e. more than half or less than half of the toenail. The growth rates of affected and unaffected great toenails of all patients were measured. Before a normal appearance was reached, the growth rates of affected great toenails, when the affected area occupied more than half of total nail plate, was slower than that of the unaffected great toenails. After a normal appearance was achieved, there were no differences in growth rates between affected and unaffected great toenails. Therefore, this study of patients with unilateral toenail onychomycosis did not support the hypothesis that slow nail growth rate is a predisposing factor for onychomycosis.