Fifty-one morbidly obese women were randomized and operated upon with either gastroplasty or gastric bypass. The patients were studied preoperatively and 12 months after surgery. The weight loss was significantly larger after gastric bypass. Fat cell weight was determined from subcutaneous fat biopsies of the epigastric, hypogastric, femoral and gluteal regions. Intra-abdominal biopsies were also taken during the operation. Fat cell weight decreased significantly in all subcutaneous regions. Fat cell number, derived from calculated body fat and measured mean subcutaneous fat cell weight, decreased significantly in both treatment groups.