The value of technetium-99m methoxyisobutylisonitrile (MIBI) single-photon emission tomography (SPET) of the chest in differentiating lung carcinomas of various histological types and benign lesions was assessed in 54 patients (47 males and 7 females aged 19-77 years) with single solid lung masses. Chest radiography had indicated that 46 of the lesions were malignant (8 small cell carcinomas, 14 epidermoid carcinomas, 18 adenocarcinomas and 6 undifferentiated large cell carcinomas) and eight, benign. Ten, volunteers who also agreed to undergo 99mTc-MIBI SPET of the chest served as a control group. The results showed that only 65% (30/46) of the lung malignancies were detected by 99mTc-MIBI SPECT, including 63% (5/8) of the small cell carcinomas, 64% (9/14) of the epidermoid carcinomas, 72% (13/18) of the adenocarcinomas and 50% (3/6) of the undifferentiated large cell carcinomas. However, 75% (6/8) of the benign lesions were also detected. The diagnostic sensitivity, specificity and accuracy in differentiating malignant and benign lesions were 65%, 57% and 70%, respectively. We conclude that 99mTc-MIBI is of limited use in the differentiation of single solid lesions in the lungs.