In lung carcinomas, the proliferative activity, as detected by Ki-67 antigen immunostaining of surgical specimens, is a valuable factor predicting clinical evolution and response to treatment. We investigated whether bronchial endoscopic and fine-needle aspiration (FNA) biopsies of lung carcinoma can provide a reliable estimation of the tumor proliferative fraction (TPF). In 66 resectable lung carcinomas, sections of preoperative bronchial or FNA biopsies and the corresponding surgical specimens were stained in parallel for Ki-67 using MIB-1 monoclonal. The mean TPF was 44.7% in the surgical specimens, 40.3% in bronchial biopsies, and 26.3% in FNAs. When the scores of biopsy and resected specimen of each individual tumor were compared, a significant correlation between the TPFs of preoperative and postoperative specimens was found (r = .79). In both biopsy and surgical specimens, a high TPF was associated with squamous cell carcinoma histological type and high-grade (poorly differentiated) tumors. In addition, a significantly (P < .05) lower disease-free interval was found in patients affected by highly proliferating tumors (irrespective of the tumor stage). We conclude that the proliferative activity of lung cancer can be reliably assessed in bronchial or FNA biopsies. This information could help to select chemotherapy protocols in nonresectable lung carcinomas.