A review and update of published studies on prognostic factors support the following conclusions: The number of axillary lymph nodes involved with tumor remains the most important prognostic factor for women with primary breast cancer. In stage I disease, the lack of estrogen receptor (ER) seems to be the most important factor for predicting earlier recurrence and poorer survival. In stage II breast cancer, progesterone receptor (PgR) content appears to be better than ER content in predicting disease-free and overall survival. Measurement of proliferative activity (S-phase DNA) by thymidine labeling or flow cytometry (FCM) and of aneuploidy by FCM also provides prognostic information. Patients with aneuploid tumors, or with high S-phase diploid tumors, are at increased risk for relapse. Amplification of oncogenes, particularly HER-2/neu, may provide additional prognostic information in combination with other established prognostic factors.