Obesity is assumed to be a risk factor in the occurrence of thrombophlebitis. We studied 168 consecutive patients retrospectively; 33 were men and 135 women, with an average age of 34 (range 27 to 41) years. All patients had a gastric bypass because of obesity, with a minimum of 100 lb over normal weight. The mean weight was 279.2 lb (range 191 to 500). Only three patients had a history of deep vein thrombophlebitis, with no thromboembolism. Eighty-four of the patients were studied preoperatively by noninvasive means (Doppler, impedance plethysmography [IPG], phleborheography [PRG]); 12 had evidence of old disease, and two had a history of treated deep vein thrombophlebitis. No patient had prophylactic therapy. The incidence of clinical deep vein thrombophlebitis was zero; noninvasive evaluation in 64 patients demonstrated no abnormality. Postoperative thromboembolism, which occurred in three of 168 (1.8%) patients, was confirmed by ventilation-perfusion scan and pulmonary angiogram. The mortality from thromboembolism was less than 1% (1/168 patients). Extreme obesity may not necessarily constitute a major risk factor in the occurrence of postoperative deep vein thrombophlebitis and thromboembolism. Prophylactic medications and therapy may add inappropriate risk, undue cost, and unnecessary discomfort, and must be weighed against a mortality of less than 1%.