Pachyonychia congenita is a rare hereditary disorder characterized mainly by nail hypertrophy and dyskeratoses of skin and mucous membranes. A thorough literature survey since its first description in 1904 up to 1985 revealed 168 cases of pachyonychia congenita. There were no indications of any sex or ethnic group predilection. Based on this survey the following classification is suggested: type I (56.2% of cases), hyperkeratosis of nails, palmoplantar keratosis, follicular keratosis, and oral leukokeratosis; type II (24.9% of cases), clinical findings of type I plus bullae of palms and soles, palmar and plantar hyperhidrosis, natal or neonatal teeth, and steatocystoma multiplex; type III (11.7% of cases), clinical findings of types I and II plus angular cheilosis, corneal dyskeratosis, and cataracts; and type IV (7.2% of cases), clinical findings of types I, II, and III plus laryngeal lesions, hoarseness, mental retardation, hair anomalies, and alopecia.