Vitamin and mineral assays were performed on blood in 20 gastric bypass patients preoperatively and 6 and 12 months postoperatively. Values were compared with serial food records in nine patients. Postoperatively, all patients were prescribed a supplement containing the recommended dietary allowances (RDA) for vitamins and minerals. Weight, calorie and protein intake, and total serum protein decreased over the study interval (p less than 0.01). Dietary intakes of vitamins B1, B2, B6, folate, iron and zinc fell (p less than 0.01), but total intake (i.e., diet + supplement) did not decrease with the exception of iron. Blood indicators of these nutrients were normal preoperatively and did not decline. However, plasma vitamin B12 levels decreased from 385 pg/ml preoperatively to 234 pg/ml at 1 year (p = 0.0064), despite an increase in total vitamin B12 intake from 2.6 to 11.7 micrograms/day (p = 0.1173). Five patients (27.8%) had abnormally low plasma vitamin B12 levels at 1 year postoperatively; four were taking at least the RDA for vitamin B12 as supplements. Although oral supplementation containing the RDA for micronutrients can prevent abnormal blood indicators of most vitamins and minerals, it is insufficient to maintain normal plasma B12 levels in about 30% of gastric bypass patients.