Jejuno-ileal bypass surgery for morbid obesity can cause metabolic and electrolyte imbalances. Another case of severe, symptomatic magnesium deficiency after jejuno-ileal bypass surgery is described. This complication was not encountered in a prospective evaluation of the magnesium status after gastric restrictive bypass surgery. Significant (p less than 0.005) body weight decrease was not accompanied by any (serum, erythrocyte or urinary) evidence of magnesium loss for 12 months after surgery.