Fine needle aspiration on small tumour pieces from 153 premenopausal women with primary breast cancer at high risk for developing recurrent disease has been used for flow cytometric DNA analysis; 133 of the tumours had either one or more cell populations different from diploid or a single, diploid cell population with an aspiration cytology showing presence of typical tumour cells. The distribution of tumour cell populations in this material was rather unusual with a high number of tetraploid populations. There were significant differences between various DNA ploidy classes with respect to prognostic factors such as lymph node involvement and oestrogen and progesterone receptor status. The S-phase fraction was significantly lower for diploid tumour cell populations compared to aneuploid populations. Diploid tumours with high S-phase fraction were not correlated to prognostic factors usually indicating a poor prognosis. Patients with an aneuploid tumour cell population classified as medium aneuploid had a significantly shorter recurrence-free survival than the rest of the patients. No conclusion on the effect of adjuvant treatment for patients with different tumour cell populations can be made at present.