Occupational exposure to elevated concentrations of benzene is a known cause of leukemia in adults. Concentrations of benzene from motor vehicle exhaust could be elevated along highly trafficked streets. Several studies have reported significant associations between proximity to highly trafficked streets and the occurrence of childhood cancers and childhood leukemia. These associations may be due to chronic exposure to benzene or other carcinogenic components of vehicle exhaust from these nearby streets or to some other factor (e.g., noise, increased light exposure, or some unaccounted--for socioeconomic variable). We used data for homes studied in an earlier childhood cancer study conducted in Denver, CO, in the 1980s. No air pollution measurements were made in the original study. We identified the highest trafficked street near each study home and obtained the traffic density in 1979 and 1990. Traffic density was weighted for the distance from the street to the home using 3 different widths of Gaussian curves to approximate the decay of the emissions into the surrounding neighborhoods. The associations between the 750-ft-wide distance-weighted traffic density metrics and all childhood cancers and childhood leukemia are strongest in the highest traffic density category (> or = 20,000 vehicles per day [VPD]). The odds ratio is 5.90 (95% confidence interval [CI] 1.69-20.56) for all cancers and 8.28 (95% CI 2.09-32.80) for leukemia. The results are suggestive of an association between proximal high traffic streets with traffic counts > or = 20,000 VPD and childhood cancer, including leukemia.