Seven adult, morbidly obese patients scheduled for bariatric surgery were studied in an identical manner preoperatively and postoperatively. Six patients underwent gastroplasties, and one patient underwent a gastric bypass procedure. A single 250-mg dose of erythromycin as a Filmtab was administered orally after an overnight fast. Multiple venous blood samples were collected over a 12-hour period. After surgery, each patient had a decrease in peak concentration and an increase in the time to reach peak concentration compared to presurgery values. Mean peak concentration was reduced from 1.04 micrograms/ml preoperatively to 0.55 micrograms/ml postoperatively, and the mean time to peak increased from 3.9 hours to 6.7 hours. Mean weight-corrected AUC was reduced 41 per cent, with two patients having no detectable serum levels postoperatively. The results suggest that the erythromycin product evaluated is of questionable value for use in bariatric surgery patients.