Background: Atrazine is the most heavily applied agricultural pesticide for crop production in the United States. Both animal and human studies have suggested that atrazine is possibly carcinogenic, but results have been mixed. We evaluated cancer incidence in atrazine-exposed pesticide applicators among 53 943 participants in the Agricultural Health Study, a prospective cohort study of licensed pesticide applicators in Iowa and North Carolina. Methods: We obtained detailed pesticide exposure information using a self-administered questionnaire completed at the time of enrollment (1993-1997). Cancer incidence was followed through December 31, 2001. We used adjusted Poisson regression to calculate rate ratios (RRs) and 95% confidence intervals (CIs) of multiple types of cancer among atrazine exposed applicators. Ptrend values were calculated using atrazine exposure as a continuous variable, and all statistical tests were two-sided. Two exposure metrics were used: quartiles of lifetime days of exposure and quartiles of intensity-weighted lifetime days of exposure. Results: 36 513 (68%) applicators reported ever using atrazine; exposure was not associated with overall cancer incidence. Comparisons of cancer incidence in applicators with the highest atrazine exposure and those with the lowest exposure, assessed by lifetime days (RRLD) and intensity-weighted lifetime days (RRIWLD) of exposure yielded the following results: prostate cancer, RRLD = 0.88, 95% CI = 0.63 to 1.23, Ptrend =.26, and RRIWLD = 0.89, 95% CI = 0.63 to 1.25, Ptrend = .35; lung cancer, RRLD 1.91, 95% CI = 0.93 to 3.94, Ptrend = .08, and RRIWLD 1.37, 95% CI = 0.65 to 2.86, Ptrend = .19; bladder cancer, RRLD = 3.06, 95% CI = 0.86 to 10.81, Ptrend =.18, and RRIWLD = 0.85, 95% CI = 0.24 to 2.94, Ptrend = .71; non-Hodgkin lymphoma, RRLD = 1.61, 95% CI = 0.62 to 4.16, Ptrend = .35, and RRIWLD = 1.75, 95% CI = 0.73 to 4.20, Ptrend = .14; and multiple myeloma, RRLD = 1.60, 95% CI = 0.37 to 7.01, Ptrend = .41, and RRIWLD = 2.17, 95 % CI = 0.45 to 10.32, Ptrend =.21. Conclusions: Our analyses did not find any clear associations between atrazine exposure and any cancer analyzed. However, further studies are warranted for tumor types in which there was a suggestion of trend (lung, bladder, non-Hodgkin lymphoma, and multiple myeloma).