The purpose of this study was to assess the impact of prior cardiac or pulmonary disease upon the utility of ventilation/perfusion (V/Q) scans in the diagnosis of acute pulmonary embolism (PE). Ventilation/perfusion scans were evaluated among 365 patients with no prior cardiac or pulmonary disease and compared to V/Q scans in 526 patients with prior cardiac or pulmonary disease. Among patients with no prior cardiac or pulmonary disease, PE was present in 117 and PE was excluded in 248. Among patients with prior cardiac or pulmonary disease, PE was present in 140 and excluded in 386. The positive predictive value for PE of high probability V/Q scans among patients with prior cardiac or pulmonary disease, 55 of 66 (83 percent), was not significantly lower than among patients without prior cardiac or pulmonary disease, 50 of 54 (93 percent) (NS). The positive predictive value of low probability V/Q scans was similar with prior cardiac or pulmonary disease, 25 of 182 (14 percent), and without prior cardiac or pulmonary disease, 17 of 113 (15 percent) (NS), as was the predictive value of near normal/normal V/Q scans, 2 of 51 (4 percent), vs 3 of 79 (4 percent) (NS). The sensitivity of high probability V/Q scans, with pre-existing cardiac or pulmonary disease and without, 55 of 140 (39 percent) vs 50 of 117 (43 percent), did not differ significantly. The specificity of high probability V/Q scans with prior cardiac or pulmonary disease and without, 375 of 386 (97 percent) vs 244 of 248 (98 percent) was also similar (NS). In conclusion, the diagnostic utility of V/Q scans for acute PE was not impaired by the presence of pre-existing cardiac or pulmonary disease. Fewer patients, however, with no prior cardiac or pulmonary disease, had intermediate (indeterminate) V/Q scans.