DNA ploidy and S-phase fraction (SPF) were determined by flow cytometry on paraffin-embedded tumor material from 243 patients treated during 1980-1985. Patients with well differentiated and moderately differentiated tumors without solid areas (n = 351) formed a low-risk group (corrected 5-year survival 90%). Twenty-four patients, dead of disease within 5 years, were compared with 52 survivors. The estimated death risk was higher for those with SPF > or = 8.0% compared with those with SPF < 8.0% (odds ratio = 18.2; p < 0.001). SPF was the only independent prognostic factor in a multivariate analysis also including age, clinical stage and grade of differentiation. Patients with moderately differentiated tumors with solid areas or poorly differentiated tumors (n = 208) were regarded as a high-risk group. There was a difference in survival according to ploidy; the corrected 5-year survival was 75% for 106 patients with diploid tumors compared with 44% for those with non-diploid tumors (p < 0.0001). In a multivariate analysis DNA ploidy, age and clinical stage were independent prognostic factors, whereas SPF was no longer significant. Thus, DNA ploidy and SPF have different prognostic values depending on histological grade of endometrial carcinoma.