A model of cognitive ability, consisting of the verbal and performance components of both intellectual level and learning is described and it is used to help in showing the similarities and differences in the cognitive functioning of children with Duchenne muscular dystrophy (DMD) and cerebral palsy (CP). Previous studies of the intellectual functioning of boys with DMD are reviewed in detail, and it is suggested that the model proposed here can add greatly to descriptions of cognitive functioning in these boys and in those with other medical conditions. The data give support to the proposed model and show differences between the groups, which indicate the greater power of this model over measures of general intellectual functioning alone. The CP group obtained lower mean scores on measures of intellectual performance level whereas the DMD group was, on average, worse at the test of verbal learning. The relation of the cognitive model to educational attainment in these groups is also examined.