In a registry-based case-control study of breast cancer in farm labor union members in California, 128 breast cancer (BC) cases newly diagnosed in 1988--2001 and 640 cancer-free controls were investigated. Stage and grade of disease at diagnosis were about the same as in the California Hispanic population. Risk of breast cancer was not associated with work with any specific crops or commodities except mushrooms, where the adjusted odds ratio (OR) was 6.00 (95% CI 2.01-18.0). Controlling for covariates, adjusted ORs (and 95% CIs) for breast cancer in quartiles of pesticide use were 1.00, 1.30 (0.73-2.30), 1.23 (0.67-2.27), and 1.41 (0.66-3.02). Chlordane, malathion, and 2,4-D were associated with increased risk. Risk associated with chemical use was stronger in younger women, those with early-onset breast cancer, and those diagnosed earlier (1988--1994).