Seventy patients were investigated by isotope venography of the lower leg veigns and pelvis using 99Tcm human serum albumin, mainly as microspheres, in the course of routine lung scanning. In 54 of these patients, 52 pelvic, and 98 conventional ascending X-ray contrast venograms were performed for comparison, ninety three per cent showed good correlation, and 4% poor correlation. There were 2% false positive and 1% false negative isotope examinations. Isotope venography proved to be a painless, safe and reliable method for the detection of deep vein thrombosis. Its correlation with contrast venography is high, especially for the upper thigh and pelvis. It can be performed in 30 minutes, including standard lung scan, and can be repeated in patients to assess response to treatment, saving the patient the discomfort and radiation of comparable radiological examinations.