This study investigated the diagnostic efficacy and safety of a percutaneous core biopsy technique in patients with undiagnosed intrapulmonary mass lesions. Eighty-four consecutive biopsies were performed in 81 patients with intrapulmonary mass lesions. Follow-up data permitted a final diagnosis in 79 of these 81 patients, which allowed an assessment of the accuracy of the technique. Fifty-nine of 68 patients with malignant lesions were diagnosed by core biopsy (87% sensitivity) and nine of 11 with benign lesions (82% sensitivity). The complication rate was 21%. Malignant tumor cell-type was predicted correctly from biopsy material in 60% of patients. It is concluded that core biopsy is a safe procedure if restricted to the investigation of intrapulmonary mass lesions, and is effective in the identification of malignant tumors. Its chief advantage over aspiration techniques is that the tissue core obtained permits a specific diagnosis in a high proportion of patients with benign lesions, thus reducing the need in these patients for continued observation or diagnostic thoracotomy.