Cyclin B1 is a key molecule for G2-M-phase transition during the cell cycle and is overexpressed in various tumor types. However, the expression status of cyclin B1 in lung cancer and its clinical significance remain unknown. We used immunohistochemistry studies to examine the expression of cyclin B1 in 77 non-small cell lung cancer specimens from patients with histological stage I disease. All of the patients underwent curative surgical treatment. The median length of follow-up care is 8.2 years. High-level cyclin B1 expression (a cyclin B1 labeling index > or =15%) was observed in 17 of the 77 (22%) tumors. Patients whose tumors expressed a high level of cyclin B1 had a significantly shorter survival time than patients whose tumors expressed a low level of cyclin B1 (P = 0.02, log-rank test). Interestingly, overexpression of cyclin B1 was more frequently observed in tumors with squamous cell histology than in tumors with other histological cell types (P = 0.01, Fisher's exact test). A subgroup analysis revealed that cyclin B1 overexpression seems to be an adverse prognostic factor only in patients with squamous cell carcinoma (SCC) of the lung (P = 0.02, log-rank test). Our data indicate that cyclin B1 may be dysregulated in non-small cell lung cancer, particularly in the SCC subtype, and that a high level of cyclin B1 expression may be a prognostic marker for patients with early-stage SCC of the lung.