HER-2neu alterations (amplification and overexpression), proliferative activity (3H-Thymidine-Labeling Index, 3H-Tdr-LI) and tumor ploidy (flow-cytometry) were analyzed in a human breast cancer series. Overexpressed tumors showed a significantly higher median 3H-Tdr-LI than normally expressed (p = 0.04), as also did amplified/overexpressed with respect to non-amplified/normally expressed (p less than 0.04). These differences were confirmed only in N-primary tumors. In fact, high expressed cases demonstrated a median 3H-Tdr-LI of 5.5% with respect to 1.9% for low expressed cases (p = 0.04) and amplified/overexpressed had a 3H-Tdr-LI of 9% with respect to 2% for non amplified/low expressed (p = 0.009). HER-2/neu alterations were not correlated with tumor DNA content.