Node-negative breast cancer: prognostic subgroups defined by tumor size and flow cytometry Adjuvant systemic therapy for women with node-negative breast cancer is most easily justified for those patients at highest risk of relapse. We have examined the impact of tumor size, histologic grade, estrogen receptor (ER) status, tumor ploidy, and S-phase fraction (SPF) on relapse-free survival (RFS) for 169 patients with node-negative breast cancer in order to identify groups of patients at high and low risk of relapse. Patients with small tumors (less than or equal to 1.0 cm) had a significantly better RFS than those with larger tumors (P = .005), with 96% remaining relapse-free at 5 years. Patients with tumors less than or equal to 1.0 cm were thus excluded from analysis when attempting to define a group with a poor prognosis. Within the group of patients with tumors greater than 1.0 cm, tumor ploidy (P = .63), ER status (P = .3), or progesterone receptor (PgR) status (P = .24) did not predict for RFS. Patients with grade 1 or 2 infiltrating ductal tumors had a significantly better prognosis than those with grade 3 tumors (P = .04). The prognostic factor that gave the widest separation between subgroups, however, was SPF. Patients whose tumors were greater than 1.0 cm with an SPF less than or equal to 10% had a 5-year RFS of 78% compared with a 5-year RFS of 52% for those with an SPF greater than 10% (P = .006). We have combined tumor size and SPF to identify three prognostic groups: (1) tumor less than or equal to 1.0 cm, 5-year RFS 96%; (2) tumor greater than 1.0 cm plus SPF less than or equal to 10%, 5-year RFS 78%; 3) tumor greater than 1.0 cm plus SPF greater than 10%, 5-year RFS 52%. These prognostic groupings may help identify patients most suitable for adjuvant therapy. 