The purpose of this study was to estimate the relative risk of breast and colorectal cancers in women who were previously diagnosed with endometrial cancer. This study was conducted using a population-based cohort of 2347 women diagnosed with invasive cancer of the endometrium between January 1, 1984, and December 31, 1995 in Orange County, California. Only women with a diagnosis of invasive endometrial cancer at age 80 years old or below were included in the analysis (N = 2170). In this same cohort, metachronous and synchronous breast and colorectal cancers were ascertained and the risk of developing one or the other type of neoplasm was compared to the expected number of cases derived from cancer incidence in California by age, 1988-1992. We found a statistically increased risk of breast cancer as a second primary, while the observed incidence in colorectal risk did not reach statistical significance. The association between endometrial cancer and breast and possibly colorectal cancer indicates the importance of common etiologies for these cancers.