The suitability of measuring the S-phase fraction in human breast cancer by labelling tumour cells from fine needle aspirates (FNAs) in vitro with iododeoxyuridine (IdU) was studied in 11 patients. The S-phase fraction was measured both in preoperative FNAs labelled in vitro with IdU, and in FNAs taken from the same tumour when surgically removed after intravenous administration of IdU. Frozen sections were also immunostained for incorporated IdU. The mean S-phase fraction measured in FNAs after in vitro or in vivo labelling and in sections after in vivo labelling was 4.0, 3.6, and 3.1, respectively. Results of in vitro and in vivo labelling of FNAs with IdU were similar. However, as the S-phase fraction in breast cancer is generally low, the variation between the different measurements is too large; therefore, the S-phase fraction is not a suitable indicator of response to treatment.