Three successive generations of CFW mice were exposed to a 25-mT (250,000 mG), 60-Hz electromagnetic field for prolonged periods. At the end of the exposure period, animals from both the exposed and control groups were sacrificed for tests. A complete autopsy was performed and tissue sections were taken from the main organs for histopathological examination. The results from the pathological findings in the various animals were classified under the following categories: (1) normal; (2) lymphoid hyperplasia; (3) premalignant changes; (4) early lymphoma; (5) advanced lymphoma. The three first-generation animals developed generalized lymphoid hyperplasia. In the second-generation animals, 5% developed premalignant changes, and 15.8% had lymphoid hyperplasia. In addition, 4 female mice left in the field for 418 days developed malignant lymphoma. In the third-generation animals, 58% developed premalignant changes or malignant lymphoma. An additional 30% had lymphoid hyperplasia. Statistical analysis of the data using the Mantel-Haenszel test for the difference in the prevalence of lymphoma between the exposed and control groups shows a very significant difference for the male groups (P < 0.001), the female groups (P < 0.001), and all animals combined (P < 0.001). The results suggest a cause-effect relationship between chronic exposure to very strong 60-Hz magnetic field for prolonged period and the development of malignant lymphoma in CFW mice.