Cell kinetics analysis of lung carcinoma using DNA flow cytometry has shown a significant correlation with the biological behavior of these neoplasms. Ploidy has shown a more significant association with aggressive behavior. The method may however not be available in all centers. Two counts of the AgNOR silver stain have been correlated with ploidy and proliferative activity (PA). The first count, which is the mean number of AgNOR granules (mAgNOR), correlates with ploidy. The second count is the percentage of cells with ≥ 5 AgNORs/nucleus (pAgNOR), reflects PA. We performed the AgNOR silver stain using the two above-mentioned counts in 41 cases of surgically resected non-small cell carcinoma of the lung. The cases included 14 adenocarcinomas, 24 squamous cell carcinomas, and three undifferentiated non-small cell carcinomas. Follow-up data were available on 36 of the patients, ranging from 10 to 31 months (median 18 months). Thirteen of these patients (36%) developed progressive disease. Adenocarcinomas showed mAgNOR counts suggestive of aneuploidy (≥ 2.4) in nine of the 14 patients (64%) and 16 of the 24 squamous carcinomas (66%). The adenocarcinomas showed high pAgNOR counts (≥ 8%) in eight of the 14 cases (57%), in contrast to 15 of the 24 squamous carcinomas (62%). The AgNOR counts did not show any statistically significant correlation with tumor type, grade or stage of disease. The mAgNOR counts were aneuploid in all 13 progressive cases and in only 10 of the 23 stable cases (43%)(P=0.001). The pAgNOR counts were high in 12 of the 13 cases that progressed (92%), in contrast to 10 of the 23 stable cases (43%)(P=0.01). There is no significant evidence that squamous carcinoma of the lung may have a higher incidence of aneuploidy and high PA than adenocarcinoma. Our data also confirm previous data showing that aneuploid lung carcinomas have more aggressive behavior than diploid ones. This study also indicates that, despite the short-term follow-up data, the use of the AgNOR silver stain for cell kinetics analysis of non-small cell carcinoma of the lung may potentially provide useful predictive information on the biologic behavior of lung carcinoma. Long-term follow-up may provide more significant information.