The bronchus sign on CT represents the presence of a bronchus leading directly to a peripheral pulmonary lesion. We investigated the value of this sign in predicting the results of transbronchial biopsy and brushing in 33 consecutive cases of proved peripheral bronchogenic carcinoma studied with thin-slice CT (2-mm-thick sections). The bronchus sign was seen on CT in 22 patients and was absent in 11. Transbronchial biopsy and brushing showed peripheral carcinoma in 13 (59%) of 22 patients in whom the bronchus sign was seen on CT and in only two (18%) of 11 patients in whom it was not seen. The difference is statistically significant (Fisher's exact test, p = .029). When analyzed by the order of involved bronchus, a 90% success rate of transbronchial biopsy and brushing was found in patients in whom the bronchus sign was seen at a fourth-order bronchus (p = .01). This compared with a success of 33% when the bronchus sign was seen at fifth-, sixth-, or seventh-order branches. Our results suggest that the bronchus sign at a fourth-order bronchus is valuable in predicting the success of transbronchial biopsy and brushing. The presence of the sign on CT may be useful in determining if the workup should include transbronchial biopsy and brushing or transthoracic needle aspiration in patients with peripheral lung lesions.