Although farmers appear to be at an increased risk of prostate cancer, the specific exposures which produce the excess risk remain unexplained. This study was based on a retrospectively assembled cohort of male Manitoba, Saskatchewan, and Alberta, Canada, farmers age 45 years or older identified in the 1971 Canadian censuses of population and agriculture. The cohort was linked to the Canadian National Mortality Database using an iterative computer record linkage system for the period June 1971 to the end of 1987. A total of 1,148 prostate cancer deaths and 2,213,478 person-years were observed. Using Poisson regression, the study examined the relation between the risk of dying from prostate cancer and various farm practices as identified on the 1971 Census of Agriculture, including exposure to chickens, cattle, pesticides, and fuels. A weak, but statistically significant, association was found between number of acres sprayed with herbicides in 1970 and risk of prostate cancer mortality. When the analysis was restricted to farmers believed to be subject to the least amount of misclassification, the risk associated with acres sprayed with herbicides increased (rate ratio (RR) = 2.23 for 250 or more acres sprayed; 95% confidence interval (CI) 1.30-3.84; test for trend, p < 0.01). No other farm exposure examined was associated with any detectable pattern of increased or decreased risk. These findings encourage further research to examine the effects of herbicides on prostate cancer.