This is a retrospective study of stature and bone age before and during androgen substitution for hypogonadotrophic hypogonadism in 25 males. Before androgen treatment, stature tends to be low for chronological age but normal for bone age. Treatment results in normal final height. Growth in length of metacarpal bone II (MC II), measured on hand radiographs, mimics growth in height. From measurements of total bone width of MC II and combined cortical thickness of MC II, the cortical area (indicating bone mass) and the relative cortical area (indicating bone density) were calculated. Bone mass was well within the limits of a published control series, both before and during androgen treatment. Bone density, however, was, without exception, below the P50 for controls prior to androgen treatment, and increased only slightly during treatment. It is concluded that prepuberal deficiency of androgen, possibly in early infancy, causes a permanent defect in bone density which is only marginally corrected by later androgen treatment.