['The prognostic significance of cell proliferation, estimated as cytometric S-phase fraction (SPF), was investigated in node-negative breast cancer patients with small tumors (T1, NO). The 219 stage I patients originated from two series and were diagnosed either from 1978 to 1981 or from 1981 to 1985. The tumors were analyzed for estrogen receptors (ERs) by isoelectric focusing and for cellular DNA content by static cytofluorometry or flow cytometry.', 'A high SPF correlated with the absence of ERs and abnormal DNA content, and was less often found in tumors smaller than 11 mm compared with those with a diameter between 11 and 20 mm. Among the variables age, tumor size, DNA ploidy, ER status, and SPF, only SPF showed a significant association with distant recurrence and breast cancer survival in systemically untreated patients. The relative recurrence rate for patients with an SPF of 10% or greater was three times that for patients with lower SPFs. Estimated 8-year breast cancer survival rates for the same groups were 72% and 91%, respectively.'] Is s-phase fraction a prognostic factor in stage I breast carcinoma?