A new method of producing aerosols (technegas) in which 99Tcm is bound to carbon atoms (99Tcm-C) was evaluated by comparing 99Tcm-C images with those obtained with 81Krm in the same patients. Twenty-five patients with suspected pulmonary embolism (PE) were studied. Immediately after the last 99Tcm-C view, the patients remained in supine position and inhaled 81Krm at tidal volume. Immediately after the 81Krm ventilation views were recorded, 4-7 mCi of MAA were injected IV. The same four views (ant, lop, rop, post) were recorded after inhalation of 99Tcm-C and 81Krm (200 kcounts) and 99Tcm MAA injection (400 kcounts). The mean penetration index of 99Tcm-C (0.91) was lower than that of 81Krm (1.04) (P less than 0.03). The apex to base lung distribution of 99Tcm-C and 81Krm appeared to be similar. The mean heterogeneity of 99Tcm distribution was 23, greater than that of 81Krm (14) (P = 10(-4)). The 99Tcm-C ventilation image quality was considered very good for 16 patients and good for 6 others. Significant foci of high bronchial uptake were infrequent. Interpretation of the examinations performed after inhalation of 99Tcm-C and 81Krm was concordant in all cases. No patient had an 81Krm/99Tcm MAA examination suggestive of PE when 99Tcm-C/99Tcm MAA indicated a low probability of PE, and vice versa. 99Tcm-C aerosols enable good quality ventilation images to be obtained in nearly all cases. Thus 99Tcm-C aerosols could be used in preference to 81Krm in ventilation studies for the diagnosis of PE.