Bone tissue was examined in 21 patients who had undergone jejuno-ileal bypass for obesity between 1971 and 1974. 10 patients had osteomalacia with evidence of secondary hyperparathyroidism. Clinical symptoms and biochemical and radiological investigations were often unreliable in diagnosing bone disease, although plasma-25-hydroxyvitamin-D and plasma-phosphate concentrations were significantly lower and plasma-parathyroid-hormone concentrations were significantly higher in the patients with bone disease. The presence of osteomalacia was unrelated to age, length of time since bypass, or post-bypass weight-loss, and plasma-25-hydroxyvitamin-D levels did not correlate closely with bone histological changes. It is concluded that osteomalacia is common after jejuno-ileal bypass and that factors other than simple vitamin-D deficiency may contribute to its development.