The clinical features and noninvasive tests, including ventilation perfusion (V/Q) lung scans, were assessed in 108 patients with chronic obstructive pulmonary disease (COPD) suspected of having pulmonary embolism (PE). Twenty-one (19 percent) of 108 patients had PE. In the majority of patients, it was impossible to distinguish between patients with and without PE by clinical assessment alone. However, when a high clinical index of suspicion was present, PE was confirmed by angiography in three of three patients, but the V/Q scan was of intermediate probability. No roentgenographic abnormalities distinguished between PE and no PE. There was no difference between the alveolar-arterial oxygen gradients in either group, nor was there evidence of a reduction in the PaCO2 in patients with PE who had prior hypercapnia. Among the 108 patients with COPD, high, intermediate, low, and normal/near normal probability scans were present in 5 percent, 60 percent, 30 percent, and 5 percent, respectively. The frequency of PE in these V/Q scan categories was five (100 percent) of five, 14 (22 percent) of 65, two (6 percent) of 33, and zero (0 percent) of five, respectively. In conclusion, in the majority of patients, the V/Q scan diagnosis is usually intermediate and such patients require further investigational studies, including angiography. However, among the few patients who demonstrated a high probability lung scan, there was a high positive predictive value for PE effectively avoiding the need for further studies. In those patients with low probability or near normal/normal V/Q scans, the negative predictive value was not lower than the general hospital population.