Background: Observed regional differences in breast cancer incidence could provide valuable clues to the etiology of this disease. The pattern of historically higher breast cancer rates among residents of California's San Francisco Bay and Southern Coastal areas is evident in the disease experience among members of the California Teachers Study. This large cohort study has followed female professional school employees for cancer incidence since 1995 and has collected extensive information on breast cancer risk factors.

Methods: Between 1996 and 1999, invasive breast cancer was diagnosed in 1562 of the 115,611 cohort members who could be geocoded to a California address in 1995 and who had no previous breast cancer diagnosis. Adjusted hazard rate ratios (HRs) were estimated through multivariate Cox proportional hazards modeling.

Results: Rates were higher for cohort members in the San Francisco Bay area (HR = 1.22; 95% confidence interval = 1.06-1.40) and Southern Coastal area (1.16; 1.04-1.30) compared with those in the rest of California. The distributions of variables representing socioeconomic status, urbanization, and personal risk factors were consistent with higher risks for cohort members residing in the San Francisco Bay and Southern Coastal areas. Adjustment for these factors, however, did not explain regional differences in incidence, resulting in HRs that remained elevated for these 2 areas.

Conclusion: Regional differences in breast cancer incidence in this large, well-defined cohort are not easily explained by known risk factors.