The prognostic significance of DNA ploidy, DNA index (DI), and S-phase fraction (SPF) and their various combinations were studied together with 16 other clinicopathologic factors in 222 patients with operable invasive ductal breast carcinoma. The patients have been followed for a minimum of 22 years after the diagnosis or until death. Nuclear DNA content was determined by flow cytometry from paraffin-embedded tissue. Patients with DNA diploid cancer (n = 57, 26%) had better survival rate corrected for intercurrent deaths than patients with nondiploid cancer (P = 0.002), and also, a small SPF (less than or equal to 14%, calculated in 134 cases) was associated with a favorable outcome in a univariate analysis (P = 0.01). The prognostic value of the DI and SPF was increased if they were combined. The most effective combination was obtained if diploid cancers were grouped together with DNA aneuploid cancers with a DI less than 2.1 and an SPF less than 14%. This combination had considerable prognostic value in a univariate analysis (P = 0.0002) and had independent prognostic value (P = 0.04) in Cox's multivariate analysis together with the primary tumor size (P less than 0.001) in axillary node negative patients but not in axillary node positive patients. In the whole series the presence of axillary nodal metastases (P less than 0.001), high mitotic count (P less than 0.001), a large primary tumor size (P = 0.001), poorly circumscribed tumor margin (P = 0.005), and slight or absent tubule formation (P = 0.05) were the only independent prognostic factors in a multivariate analysis.