We sought to determine whether gastric surgery might be associated with metabolic bone disease in a well-characterized population, and if so to explore its etiology. Sixteen asymptomatic middle-aged men who had had partial gastrectomy with Billroth II anastomosis but no other risk factors for metabolic bone disease were compared with unoperated healthy controls. Studies included a dietary survey, biochemical tests of bone and mineral metabolism, radiographs of the spine, determinations of bone mineral content, and bone histomorphometry. The gastric surgery subjects exhibited frequent vertebral fractures and an unusual constellation of bone abnormalities characterized by decreased bone mineral content and hyperosteoidosis without evidence of osteomalacia. Although serum immunoreactive parathyroid hormone and 25-hydroxyvitamin D levels were not different, 1,25-dihydroxyvitamin D levels were significantly higher (p = 0.037), and 24,25-dihydroxyvitamin D levels were significantly lower (p less than 0.0001) in subjects than in controls. Partial gastrectomy with Billroth II anastomosis may be associated with asymptomatic but clinically important metabolic bone disease. The pathophysiology is uncertain, but appears to involve alterations in vitamin D metabolism.