Background: Objective assessment of pain-related behaviours is crucial in adults with profound intellectual and multiple disabilities. No standardised pain observation instrument that specifically focuses on this target group is currently available. A previous study identified 12 pain-related behaviours in these persons. This study aimed to develop a valid and reliable instrument based on these 12 pain-related behaviours to assess pain in these adults in daily situations. Method: Videos of 91 adults with profound intellectual and multiple disabilities in potentially painful daily situations were scored using the 12 pain-related behaviour items. Mokken scale analysis was used to determine the construct validity of these items. Results: Analysis resulted in a 9-item scale (H =.46, Rho =.70), which appeared to be sample independent for the variables of sex, age and level of motor functioning. Conclusion: A scale containing 9 items (APOS-PIMD) with reasonable construct validity and sufficient reliability was developed, which can be used to measure pain in adults with profound intellectual and multiple disabilities in daily situations.