In a 10-year incidence survey of breast cancer in the City of Boston, 14 cases in American Chinese women were observed, while approximately 13 were expected on the basis of age-specific incidence in the white population. In Hawaii, on the other hand, while breast cancer incidence rates for ethnic Chinese are approximately three times as high as those of indigenous Chinese populations, data indicate that they are still 35% lower than those of whites in Hawaii or on the US mainland. We have compared estrogen concentrations in the urine of Chinese women in the Orient, Honolulu, and Boston. Levels of estrone (E1) and estradiol (E2) increased greatly from the low to the high risk group. The increase was evident among both the younger (15-19 years) and the older (20-24 years) women, in both follicular and luteal phase specimens. The crude unweighted average increase from Oriental to continental US Chinese was about 38% and 34% for E1 and E2, respectively. On the other hand, levels of E3 decreased from the low-risk to the high-risk group, but the decrease was modest (16%) and not always regular. There were no systematic or significant differences in the frequency of ovulation in the three groups of women. These results are compatible with the hypothesis that high levels of estrone and estradiol are important to breast carcinogenesis. Whether E3 has some additional "protective" effect or is irrelevant is unclear. The data do not support the hypothesis that total urine estrogens (E1 + E2 + E3) are predictive of population groups at high risk for breast cancer and they appear inconsistent with the hypothesis that frequency of ovulation (or anovulation) is an important aspect of breast carcinogenesis.