DNA index (ploidy) and S-phase fraction (SPF) were measured by flow cytometry in 131 invasive stage I and II breast carcinomas. Ploidy showed a strong correlation with SPF (p = 0.0001), with aneuploid tumors having a high SPF. Both cytometric parameters correlated with tumor size and hormonal receptor status. Smaller tumors tended to be diploid and have low SPF. Nodal status did not demonstrate an association with cytometric findings. There was a highly significant connection between tumor grade, especially nuclear grade, and SPF (p = 0.0001). The study demonstrates the relationship between conventional prognostic factors, DNA content, and proliferative activity of breast tumors.