Wechsler Aandult Intelligence Scale and Halstead neuropsychological performances were investigated in three groups of adult subjects with major motor epilepsy classified according to low, moderate, and high seizure frequency. The groups were equated on seizure duration and age at onset variables. On 7 of the 14 psychological measures employed, significant between-group differences were obtained, with poorest, intermediate, and best test scores associated with the high, moderate, and low seizure frequency groups, respectively. Similar performance comparisons were made on two additional groups: a "high risk" group characterized by early onset age, long seizure duration, and high seizure frequency, and a "low risk" group characterized by late onset age, short seizure duration, and low seizure frequency. Highly significant differences were obtained between the high risk and low risk groups on 12 of 14 variables, with the high risk subjects demonstrating severe cognitive and adaptive impairment in comparison to the low risk subjects. The results support the conclusion that increased frequency of major motor seizures is associated with increasing levels of adaptive impairment, and that the probability of severe cognitive-adaptive dysfunction is further and substantially increased when high seizure frequency is accompanied by a history of early onset and long duration of the disorder.