Plasma 25-hydroxyvitamin D concentrations and bone histomorphometry were investigated in 24 grossly obese subjects. The mean plasma 25OHD concentration was significantly lower in the obese group than in age-matched, healthy controls. Subnormal values were found in four obese subjects and in a further two subjects, who were investigated at the end of the summer, plasma 25-hydroxyvitamin D levels were at the lower end of the normal winter range. Bone histology was abnormal in two patients. In one, mild osteomalacia and secondary hyperparathyroidism were present while in the other patient the appearance suggested increased bone turnover, possibly as a result of healing osteomalacia. We conclude that gross obesity is associated with an increased risk of vitamin D deficiency, probably because of reduced exposure to uv radiation. Histological evidence of metabolic bone disease may also occur. Preoperative vitamin D deficiency may contribute in some patients to the development of metabolic bone disease after intestinal bypass.