The influence of jejuno-ileal bypass surgery on mineral metabolism was studied in patients with morbid obesity before operation, and 2 and 5 years after-wards. When calcium and potassium were orally substituted post-operatively, in serum calcium fractions, parathyroid hormone, phosphate, potassium and the bone mineral content remained unchanged, while serum magnesium decreased. Serum 25-hydroxyvitamin D was already low before bypass operation, and did not change thereafter. Post-operatively, the urinary excretion of oxalate rose into the upper normal range, while that of calcium, magnesium and citrate was markedly reduced. The urinary activity product of calcium oxalate rose slightly, but that of brushite decreased. Since these changes were manifest in the urine of all patients, the reasons for the post-operative formation of renal stones in 4 of 19 patients remain unclear at the moment. We conclude that the recommendation for precise oral calcium substitution after jejuno-ileal bypass operation seems justified in order to avoid serious disturbances of calcium metabolism in the long term, and to reduce intestinal oxalate absorption.