The influence of endogenously produced oestrogen on the growth of endometrial carcinoma was studied in postmenopausal 128 patients with endometrial adenocarcinoma. Plasma concentration of oestrone (E1) and oestradiol (E2) showed wide variations. Hormone levels were analysed in relation to growth rate, expressed as S-phase rate measured by flow cytometry, and to ploidy level. When the whole unclassified group was studied, no statistical relationship between E1 and E2 levels and S-phase rates were found. However, when peridiploid tumors (1.8-2.2 c) were divided according to histopathological grades, well differentiated tumors with low oestradiol concentrations (less than 60 pmol/I) had significantly lower S-phase rates than those with higher oestradiol levels (p less than 0.01). Aneuploid tumors showed high S-phase rates regardless of plasma oestradiol concentrations.