A clinical comparison of the effects on pulmonary arterial pressure induced by contrast media with various osmolalities, iohexol 140 mg I/ml (300 mosm/kg H2O), iohexol 300 mg I/ml (690 mosm/kg H2O), and diatrizoate 292 mg I/ml (1480 mosm/kg H2O) following selective pulmonary angiography was made in 12 patients with normal pulmonary arterial pressure. A double-blind crossover study was performed and the contrast media were administered in random order. The pulmonary arterial pressure was recorded continuously before, during, and for 3 min after the injection. The effect of iohexol 140 on the pulmonary arterial pressure was significantly less marked than that of diatrizoate 292, whereas no statistical significance was shown between iohexol 140 and iohexol 300. These results indicate that iso-osmolar contrast medium (iohexol 140), as well as iohexol 300, would be better tolerated than diatrizoate 292, and is therefore a safer contrast medium for selective pulmonary angiography.