Nearly 30,000 individuals ages over 21 years are diagnosed with leukemia each year in the United States. Other than benzene, radiation, and chemotherapy, which account for a small proportion of cases, there are few identified risk factors for adult leukemia. Although recent data from animal studies indicate a potentially protective role for dietary restriction in leukemogenesis, few data exist on dietary relationships in adult leukemia. Food frequency data collected at baseline (1986) were analyzed from the prospective Iowa Women's Health Study to begin to address the role of diet in adult leukemia. Data from 35,221 women ages 55-69 years were analyzed. A total of 138 women developed leukemia during the 14-year follow-up period of 1986 to 1999. With the exception of an inverse association (P trend = 0.08) with increasing consumption of all vegetables (relative risk, 0.56 and 95% confidence interval, 0.36-0.88; relative risk, 0.69 and 95% confidence interval, 0.44-1.07 for medium and high consumption, respectively), there was little evidence of an important role for other dietary factors in leukemogenesis. Analyses that excluded cases diagnosed in the first 2 years from baseline did not notably alter the results. Leukemia subgroups, including acute myeloid leukemia and chronic lymphoblastic leukemia, were also analyzed, but no statistically significant associations with dietary factors were revealed. This study provides evidence that increased vegetable consumption may decrease the risk of adult leukemia. However, given that our study focused on older women from a defined geographical area, analyses of prospective studies in other populations are needed to confirm or refute these results.