2851 radiodiagnostic examinations of the skull made in a psychiatric hospital between 1958 and 1977 were reviewed for enostosis frontalis. Out of 1398 female patients there were 125 cases of enostosis frontalis, whereas out of the 1453 male there were only 5. Hence, in females, this is a frequent condition (9%) and shows no correlation with manifest hormonal disease or any particular neuro-psychiatric disturbance. Enostosis frontalis in females can be considered to be an inconstant secondary sex characteristic. In male patients this condition is extremely rare (0.32%). The case histories of the male patients are reported. Endocrine disturbances, cerebral dysplasia and psychiatric disease in connection with mental retardation seem to be associated with enostosis frontalis in men.