A recent laboratory investigation found that female rats fed aspartame developed more lymphomas and leukemias than controls, in a dose-dependent manner, starting from a dose that may be relevant to human intake (as low as 20mg per kg body weight). Another study previously suggested a potential link to brain cancer based on an animal experiment and ecological correlations. We examined aspartame-containing beverage consumption in relation to incident hematopoietic and brain cancers among 340,045 men and 226,945 women aged 50-69 years in the NIH-AARP Diet and Health Study. The self-administered baseline food frequency questionnaire queried regarding consumption frequency and “diet” type preference of three potentially aspartame-containing beverages (soda, fruit drinks, and iced tea) as well as aspartame added to coffee and hot tea. From the responses, we computed daily consumption of aspartame, taking into account aspartame content, portion size, and intake frequency of each beverage. Relative risks (RR) and 95% confidence intervals (CI) were estimated using Cox proportional hazards regression that adjusted for age, sex, ethnicity, body mass index (BMI: weight in kilograms / height in meters squared; 4 categories of ≥18.5 and <25, ≥25 and <30, ≥30 and <35, and ≥35), and history of diabetes and smoking. During up to 5 years of follow-up (1995-2000), histologically-confirmed hematopoietic cancers (N = 2,106) and brain cancers (N = 376) were ascertained by linkage with state cancer registry of eight study areas. Compared with no consumption of aspartame-containing beverages, increasing levels of consumption were not associated with any risk of overall hematopoietic cancer (adjusted RR for ≥ 600mg/day vs. none = 0.93, 95% CI = 0.72, 1.19, P trend = 0.75) or brain cancer (RR for ≥ 400mg/day vs. none = 0.74, 95% CI = 0.49, 1.13, P trend = 0.03), in men or women. The association remained null for main subtypes of lymphoid cancers (Hodgkin lymphoma, non-Hodgkin lymphoma, and multiple myeloma) and the non-Hodgkin lymphoma subtypes (small lymphocytic lymphoma and chronic lymphocytic leukemia, immunoblastic lymphoma and lymphoblastic lymphoma/leukemia) and non-lymphoid leukemias reported in the previous animal study. These findings did not change materially with further adjustments for education, family history of cancer, physical activity, alcohol, caffeine, and other dietary and lifestyle risk factors. Our findings from this epidemiologic study suggest that consumption of aspartame-containing beverages does not raise the risk of hematopoietic or brain malignancies.