This case-control study explores the underlying mechanisms of imitation problems in boys with autism by manipulating imitation task variables and by correlating imitation performance with competence on general motor tests (Movement Assessment Battery for Children and Peabody Developmental Motor Scales). Fifty-five boys participated in this study: eight low-functioning with autism (LFA), 13 with mental retardation (MR), 17 high-functioning with autism (HFA) and 17 typically developing (TD). LFA performed significantly worse than MR on the motor test and on all imitation tasks. HFA performed significantly worse than TD on the motor test, but not on imitation tasks, with the exception of non-meaningful gestures. This study supports the notion that mainly perceptual-motor impairment, and not a cognitive weakness of symbolic representation, causes imitation problems in autism. In addition, in boys with autism, general motor as well as imitation abilities were impaired. We suggest that imitation ability has to be assessed in conjunction with motor competence. (Contains 2 tables.)