In order to assess the value of the measurement of the physiologic dead space (VD) to tidal volume (VT) ratio in pulmonary embolism (PE), a prospective study was performed in hospital inpatients suspected to have PE (n = 110; mean age +/- SD, 52.2 +/- 15.5 yr). In 16 of 29 patients in whom the diagnosis of PE was excluded on the basis of a normal radioisotope perfusion scan and/or normal pulmonary angiogram, VD/VT was less than 40%; in the other 13 patients, a VD/VT greater than 40% was associated with an abnormal spirogram. In all patients in whom PE was angiographically diagnosed (n = 16), VD/VT was greater than 40%. In the remaining 65 patients, a high probability of PE was rarely (6%) associated with a normal VD/VT, whereas in patients with a low probability of PE, 71% had normal VD/VT values. These data indicate that a VD/VT value of less than 40% makes the diagnosis of PE extremely unlikely, whereas VD/VT value greater than 40% in the presence of a normal spirogram is highly suggestive of PE. The diagnostic sensitivity of a VD/VT greater than 0.4 with a normal spirogram as a positive test of PE, and a VD/VT less than 0.4 excluding the diagnosis of PE was 100%, whereas the specificity was 94%; applying Bayesian analysis, the probability of a correct diagnosis of PE using these criteria in a similar population would be 90.5%, and of excluding PE, 96.7%. Thus, as a diagnostic test in PE, VD/VT measurement is comparable, in terms of sensitivity and specificity, to radioisotope lung scanning.