Relationship between epidermal growth factor receptor (EGFR) status and various prognostic factors was investigated in 91 human breast cancer tissues. Epidermal growth factor receptor was measured by biochemical competitive binding assay using iodine 125 epidermal growth factor (125I)-EGF. The EGFR status was not correlated with axillary lymph node involvement, tumor size, stage, and histologic type, but significantly correlated with histologic grading (P less than 0.05) and lymphatic invasion (P less than 0.01). Between EGFR and estrogen receptor (ER) status, a clear inverse relationship was observed (P less than 0.01). The Ki-67-positive stained cell rate, which reveals the proportion of cycling cells, was significantly higher in EGFR-positive tumor tissues than in EGFR-negative cases. Furthermore, preliminary postoperative survey demonstrated a high tendency of recurrence rate of patients with EGFR-positive tumors as compared with those with EGFR-negative tumors. These data suggest that EGFR status may be important for the prediction of biologically high malignant potential.