Cyclin D1, like p16INK4 (p16) and retinoblastoma (RB) proteins, participates in the cell cycle control at the G1-S transition. We have previously demonstrated altered p16 and RB protein status in non-small-cell lung cancers (NSCLCs) and their potential synergistic effect with altered p53 protein on proliferative activity (Kinoshita et al (1996) Cancer Res 56: 5557-5562). In the present study, cyclin D1 expression was studied by immunohistochemistry in the same cohort of 111 resected NSCLCs as in our previous study, and the amount of the cyclin D1 gene was analysed by Southern blot analysis in 29 NSCLCs. Cyclin D1 expression was analysed in relation to the status of p53, p16 and RB proteins, and proliferative activity determined by the Ki-67 index. It was also analysed in relation to survival of 77 patients with NSCLCs which were potentially curatively resected between 1990 and 1995. We found that: (1) cyclin D1 was expressed in 13 (11.7%) of 111 NSCLCs; (2) the cyclin D1 gene was neither significantly amplified nor rearranged; (3) cyclin D1 expression significantly correlated with altered p53 protein expression (P = 0.04), whereas it did not correlate with p16 and RB protein status; (4) proliferative activity tended to be higher in cyclin D1-positive (+) tumours than in cyclin D1-negative (-) tumours, although this difference was not statistically significant (P = 0.08); and (5) patients with cyclin D1+ tumours survived longer than patients with cyclin D1- tumours (5-year survival rates, 89% and 64% respectively, by the Kaplan-Meier method; P = 0.045 by the log-rank test), and cyclin D1 expression tended to be a favourable prognostic factor (P = 0.08 in univariate analysis). These findings suggest the involvement of cyclin D1 in the development and progression of NSCLCs, their proliferative activity and clinical outcome of NSCLC patients.