BACKGROUND: Flow cytometric study has been used to measure the DNA content of solid tumors for the last decade. DNA ploidy is an important property commonly measured by flow cytometry. The possibility to study archival paraffin-embedded tumors has hastened an appreciation of prognostic utility of this method. The aim of this study is to look for biologic prognostic indicator for survival time of patients with small cell carcinoma of lung in addition to the well known clinical prognostic factors. METHOD: DNA ploidy was measured by flow cytometric method using tumor cells isolated from paraffin embedded tissue. To evaluate the prognostic significance, DNA ploidy of small cell lung cancer was analysed in 42 patients who died after receiving anticancer chemotherapy. RESULTS: 1) Mean survival time of all patients was 190(+/-156) days. Survival time was shortened, when TNM stage and PS scale were advanced. 2) 62% of all patients was DNA aneuploidy. DNA ploidy had nothing to do with advance of TNM stage and PS scale. 3) Mean survival time of aneuploid tumor was significantly shorter(138+/-90 days) than that of diploid tumors(272 +/- 197 days).(p <0.001) 4) To exclude the influence of clinical prognostic factors such as TNM stage and PS scale, the analysis was restricted to subgroups of identical stage. We were able to find the same tendency. CONCLUSION: DNA ploidy is an independent prognostic factor in small cell lung cancer.