Atonic seizures are part of some childhood epilepsy syndromes and can also result from acquired central nervous system insult. Of 93 survivors of childhood cancer with uncontrolled seizures, 10 (11%) with atonic seizures form the basis for this report. Seven survived acute leukemia, and three survived brain tumors. The median age at cancer diagnosis was 9 months (range 2-52). Nine received systemic and six intrathecal chemotherapy; cranial irradiation was given to seven. All have learning difficulties, abnormal brain magnetic resonance images, and an abnormal electroencephalogram; six are developmentally delayed. All have difficult-to-control seizures, but six improved on myoclonus-specific drugs accelerated development in two younger patients. Atonic seizures in cancer survivors are associated with difficult-to-control seizures and pervasive cognitive impairments. Young age at cancer diagnosis and cranial irradiation may predispose the patient to development of this syndrome. Recognition and appropriate treatment of atonic seizures in cancer survivors could improve seizure control, neurologic development, and quality of life.