p16INK4 protein (p16) and retinoblastoma protein (pRB), like p53 protein, are important tumor suppressors that regulate the cell cycle. We immunohistochemically examined fresh-frozen specimens of 114 resected non-small cell lung cancers (NSCLCs) for loss of p16 and pRB expression, together with aberrant accumulation of p53 protein and the proliferative activity determined by the Ki-67 index. Three pRB-positive tumors were uninterpretable for p16 status. Of the remaining 111 tumors, 30 (27%) lacked p16 expression, and 10 (9%) lost pRB expression. No tumors showed coincident loss of both proteins, supporting the hypothesis that they function in a single pathway. Of 25 tumors, including 4 p16-negative tumors, examined by Southern blot analysis, only 2 p16-negative tumors were considered to have reduced gene dosage consistent with possible homozygous deletion of the CDKN2 gene encoding p16, suggesting that immunohistochemistry is a sensitive and suitable method to screen for p16 alteration. Loss of p16 expression did not correlate with any clinical factors or p53 status, whereas loss of pRB expression correlated with heavy smoking (P = 0.03 by Fisher's exact test and P = 0.01 by the multivariate logistic regression analysis). Proliferative activity was considerably higher in p53-positive tumors than in p53-negative tumors (P < 0.001). Loss of p16 or pRB expression was associated with a further increase in proliferative activity in the p53-positive tumors (P = 0.009) but not with proliferative activity in the p53-negative tumors. These results suggest that alteration of the p16/pRB pathway is relatively frequently involved in the development and progression of NSCLCs and that its effect on the proliferative activity is potentially synergistic with altered p53 protein.