In a prospective study on 516 breast cancer patients flow cytometry DNA ploidy and number of cell populations (defined as number of DNA stem lines) detected in the primary tumor were evaluated for prognostic purposes. The median follow-up time was about 5 years. In the 241 node negative cases, those patients with three or more cell populations had the worst prognosis, with a distant recurrence-free survival rate of about 60% at five years compared to 90% in cases with only one cell population detected in the primary tumor. The number of tumor involved axillary lymph nodes was the outstanding prognostic indicator which was confirmed in 275 node positive patients; DNA ploidy and number of cell populations did not give any significant prognostic information in this group of patients.