Jejunoileal bypass surgery is fraught with many longterm complications, among which is hypovitaminosis D. The relationship, if any, of hypovitaminosis D to the skeletal disease which may occur following this operation is, however, unknown. Consequently, we studied eight patients with low circulating levels of 25-hydroxyvitamin D who had undergone Jejunoileal bypass at least two and one-half years previously. Despite the absence of skeletal symptoms, the bone biopsies of six of these patients were abnormal. The volume of trabecular bone was diminished in the group as a whole, and half the patients had an excess of unmineralized skeletal matrix. However, no noninvasive diagnostic technique identified those patients with skeletal disease. We therefore conclude that recognition of those Jejunoileal bypass patients potentially at risk to develop clinically significant bone disease requires biopsy of the skeleton.