18 cases of post-gastrectomy osteodystrophy are described. Twelve cases were observed in ordinary hospital practice in the course of 4 years, and 6 more cases were discovered after examination of 194 cases who had undergone gastrectomy 9 years previously. The estimated incidence of osteodystrophy is 8% after 9 years. Osteoporosis alone was found in 3 cases; osteomalacis alone in 9; and a combination of osteomalacia and osteoporosis in 6. There was evidence of secondary hyperparathyoidism in one case. Osteodystrophy seemed most likely to develop in middle-aged elderly women who had had a Polya type gastrectomy for gastric ulcer. Most patients were underweight but steatorrhea or anemia were not always present even in severe cases. Adequate calcium intake did not prevent the condition. Post prandial acidemia was demonstrated in 6 cases. The possible roles of duodenal bypass, steatorrhea, intestinal hurry and post prandial acidosis in the pathogenesis of gastroprival osteodystrophy are discussed. Chelation of calcium by undigested food and hyper-calcuria due to systemic acidosis are suggested as the most important factors. All cases improved when given calcium and vitamin D in large doses. Attention is drawn to the need for a greater awareness of the existence amd prevalence of the condition.