15-O-labeled carbon dioxide (C15O2) was used for the detection of preoperative and postoperative pulmonary emboli in seventy-three patients, more than forty years old, who had undergone a total hip replacement. When the C15O2 scan was suggestive of pulmonary embolism, it was followed by conventional perfusion and ventilation (V/Q) scans. If these also suggested the presence of embolism, pulmonary angiography was done. Three different prophylactic agents against venous thromboembolic disease had been used in the seventy-three patients. Seventeen (23 per cent) of the seventy-three patients had a positive C15O2 pulmonary scan followed by a positive ventilation-perfusion scan and a positive pulmonary angiogram that showed pulmonary emboli. Fourteen (83 per cent) of these patients with proved pulmonary emboli were asymptomatic. Fifteen additional patients also had a positive C15O2 scan. However, twelve of them had either negative ventilation-perfusion scans or chest radiographs that demonstrated atelectasis, and did not have a pulmonary angiogram. Three patients with a positive C15O2 scan had a negative pulmonary angiogram. Two of them had a ventilation-perfusion scan that was interpreted as showing probable pulmonary emboli, and the remaining patient did not have a ventilation-perfusion scan. Forty-six per cent of the patients with positive C15O2 scans did not have pulmonary emboli. This sensitive and readily repeatable test provides an excellent means of detection of silent emboli, an improved method of evaluating the efficacy of prophylactic agents, and the opportunity to correlate the site, size, and onset of deep-vein thrombosis with pulmonary emboli in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)