Morphometric and bone density studies were performed on bones from 10 healthy adult dogs and 9 dogs that had exhibited signs of intestinal malabsorption for variable periods of time. The dogs with malabsorption syndrome had a marked decrease in the amount of trabecular bone in lumbar vertebrae (P less than 0.001). Evaluation of undecalcified rib cross sections revealed these dogs had a decrease in trabecular thickness (P less than 0.01) and cortical area (P less than 0.01). There was a decrease in the proportion of the trabecular surface covered with osteoblasts (P less than 0.01) and an increase in resting resorption surface (P less than 0.01) and trabecular specific surface (P less than 0.01). Three of the dogs with malabsorption syndrome and all control dogs were labeled with oxytetracycline prior to sacrifice. The dogs with malabsorption syndrome had a decreased number or complete absence of labeled bone formation sites when compared to controls. No difference was found in bone mineral density in the malabsorption cases as a group, although 1 of the dogs that had an increase in percent osteoid volume and percent osteoid-covered surface had lower ash/ml in trabecular bone samples from lumbar vertebrae. Parathyroid gland weights were available for 6 of the 9 dogs, and these were not significantly different from controls. The findings indicate that decreased bone formation, probably due to the poor nutritional status associated with malabsorption, was an important factor in the development of osteopenia.