A hospital-based case-control study of breast cancer risk related to organochlorine (OC) exposure was conducted in a multiethnic setting in New York City. We enrolled 175 breast cancer patients and 355 control patients. The overall racial/ethnic distribution was 57% Caucasian, 21% Hispanic, 22% African-American; cases and controls were frequency-matched by age and race/ethnicity. Tumor markers (estrogen and progesterone receptors, p53, erbB-2) were assessed and organochlorines (DDE, DDT, trans-nonachlor, and higher (HPCB) and lower (LPCB) chlorinated biphenyls) were measured in blood serum. Tumors among minority women were of slightly higher stage than among Caucasians, but tumor markers were similar across the racial/ethnic groups. DDE levels were highest among African-American and Hispanic women; DDT was highest among Hispanics; HPCBs were highest among African-Americans; LPCBs were lowest among Hispanics; and trans-nonachlor was highest among African-Americans. However, OC levels were not associated with risk for breast cancer, nor did OCs differ with respect to tumor stage or tumor markers. Higher DDE levels were associated with increasing body mass index (BMI), but with decreasing level of education, frequency of nulliparity, and frequency of family history of breast cancer. HPCB levels decreased with BMI and were not correlated with breast cancer risk factors. These relationships can be attributed to historical patterns of exposure and to metabolic differences in OCs related to BMI.