Background: There are no psychometrically validated instruments available to measure motor skills of children with profound intellectual and multiple disabilities (PIMD). In this study, the construct validity and reliability (Rho) of the Motor Development List for children with PIMD (MDL-PIMD), a proxy-reported measure, was investigated. Methods: The MDL-PIMD was filled in by parents of 52 children with PIMD every six months over a period of two years. The items of the original MDL were categorised into subscales. A Mokken scale analysis for polytomous items was conducted for five subscales (Posture, Locomotion, Reaching, Grabbing and Facial Actions). Results: Several items were deleted to improve scalability. All scales showed good scalability (range scale H 0.66-0.78) and reliability (range Rho 0.92-0.98). Conclusions: The MDL-PIMD seems to be a valid and reliable instrument for assessing the motor skills of children with PIMD, although more research is needed to confirm the factorial structure.