A positive association has been reported between elevated tissue organochlorines (p,p'-DDT/p,p'-DDE, PCBs, dioxins) and breast cancer in some case-control studies and occupational cohort studies. We previously reported high serum levels of p,p'-DDT and its metabolite p,p'-DDE in women living throughout Vietnam. We report here the results of a small hospital-based case-control study examining the association between blood levels of p,p'-DDT/p,p'-DDE and the risk of invasive breast cancer among residents of the north of Vietnam-an area where insecticides such as p,p'-DDT have been heavily used in the recent past. The study was conducted among patients admitted to a single hospital in the capital city of Hanoi in 1994. Study subjects were 21 women newly diagnosed with invasive adenocarcinoma of the breast, who served as cases, and 21 women of similar age with fibrocystic breast disease, who served as controls. No increase was evident in the relative risk of breast cancer with increasing tertiles of serum concentration of the compounds of interest, even after adjustment for major potential confounders, such as age at menarche, parity, history of lactation, and body weight. These results suggest that recent and past exposure to p,p'-DDT does not play an important role in the etiology of breast cancer among women living in a country with a tropical climate where insecticide use for mosquito control is common.