Objective: The aim of this study was to evaluate prognostic importance of cell ploidy and proliferation activity in non-small cell lung cancers. Survivals were compared according to the following factors: sex, age, histology, grading, DNA ploidy, tumour size, T factor, N factor and operative procedure. Methods: In a group of 191 patients in whom cytofluorometric examinations had been performed on archival tumour specimens, postoperative recurrences were observed. Results: Postoperative recurrence was observed in 64 (64.6%) of 99 patients with aneuploid tumours and in 35 (38.0%) of 92 with diploid tumours (P<0.001). Overall survival (OS) rates for the group of 92 patients operated for diploid non-small cell lung cancer (NSCLC) at 5 and 10 years were 62 and 51.1%, whereas of other 99, operated for aneuploid tumours 33.3 and 25.9%, respectively (P<0.001). In the former group of patients disease-free survival (DFS) rates at 5 and 10 years were 58.7 and 51.4% but in the latter 29.3 and 26%, respectively (P=0.00014). Significant differences dependent on cell ploidy were also observed in OS and DFS rates of patients operated respectively for SCLC (P=0.0029; P=0.00318) and adenocarcinoma (AC; P=0.0241; P=0.02109). In general, the mean percentage of S-phase cells in non-small cell lung cancers was 14.0% (SD=13.1) in patients who survived 5 years, and 22.4% (SD=15.7) in those who had a recurrence or died (P<0.001). Conclusions: In our opinion the most important finding of our work is that determination of cell ploidy in NSCLC provides a valuable supplement to the TNM stage when evaluating late results of the surgical treatment. However, the paper demonstrates that aneuploidy, although unfavourable, is not an independent prognostic factor in the group of patients with NSCLC and in the subgroups – both with squamous cell carcinoma and adenocarcinoma. Our results show also that the percentage of S-phase cells is an independent, unfavourable prognostic factor in patients treated surgically for non-small cell lung cancer and in the subgroup with squamous cell lung carcinoma.