We examined the significance of plasma HER-2/neu as a clinical or biological marker for assessing the progression of breast cancer in African American and Hispanic women with similar socioeconomic status, similar health insurance, and similar access to health care delivery. Base line studies show the following: average age of our breast cancer patients was 48 for Hispanic and 53 for African American women. Most of our patients presented invasive ductal carcinoma, and there was no ethnic difference. A larger number of Hispanic women had stage III/IV disease at the time of diagnosis. There was no significant difference in the number of African American or Hispanic patients with ER positive or negative receptors. However, a larger number of Hispanic women had PR positive tumors, and a larger number of African American women had PR negative tumors. In general, there was no difference in the levels of HER-2/neu between the two ethnic groups. Patients with tumors >5 cm had elevated plasma HER-2/neu. However, there was no ethnic difference between tumor size and HER-2/neu levels. In addition, regional node status had no impact on plasma HER-2/neu. Patients with stage III/IV tumors had elevated plasma HER-2/neu. No ethnic difference was observed at either stage I/II or III/IV. ER positive or negative status had no significant impact on plasma HER-2/neu in either ethnic group. In contrast, PR positive patients showed elevated plasma HER-2/neu. Plasma HER-2/neu (>60 U/ml) was the strongest predictor of overall survival, visceral site metastasis, and local recurrence.