A morphometric study was performed on undecalcified sections of trabecular bone from the ribs of adult dogs that were controls (n = 8), or had various degrees of renal failure as a result of perinatal irradiation (n = 16). In a group identified as markedly uremic there was an increase in the proportion of surface with osteoid seams (P less than 0.01), osteoblasts (P less than 0.01) and osteoclastic resorption (P less than 0.01) as well as the % osteoid volume (P less than 0.01). Aside from a decrease in the % trabecular bone which was of unclear significance, the results agree well with those of similar studies in man. Within this group, 6 of the samples had a histologic pattern that was principally that of hyperarathyroidism and in the remaining 2 there was evidence of a mineralization defect and osteomalacic changes. These 2 dogs had the poorest renal function and one that had been biopsied 5 months earlier, when its renal function was less impaired, had had a hyperparathyroid pattern.