Objectives: We used readily accessible, existing data to assess whether or not geographic variation in breast cancer incidence rates in the San Francisco Bay Area was related to the unequal distribution of known breast cancer risk factors.

Methods: Cancer registry and 1990 census block-group data were used to look at the associations between breast cancer incidence and known risk factors (including parity, urban/rural status, and socioeconomic indicators) in 25 California counties. Average annual age-adjusted invasive breast cancer incidence rates were calculated for the period 1988-1992, and adjusted morbidity ratios were computed.

Results: While breast cancer incidence in Marin County was 9 percent higher than that of the other 24 counties combined (relative risk = 1.09, 95 percent confidence interval = 1.01-1.18), this increase appeared to be due to the unequal distribution of known risk factors. Block-groups that had a high level of any risk factor had higher incidence rates, regardless of geographic location. After multivariate adjustment, breast cancer incidence no longer differed between Marin and the other counties (adjusted morbidity ratio = 1.02).

Conclusions: The results suggest that the unequal distribution of known risk factors was responsible for Marin County's high breast cancer incidence rate.