Evaluating pain in adults with intellectual and developmental disability (IDD) is a challenge. The Non-Communicating Adults Pain Checklist (NCAPC) was recently developed from the Non-Communicating Children's Pain Checklist (NCCPC) and examined in a group of adults with IDD (N = 228) and found to hold satisfactory construct validity, internal consistency and sensitivity to pain. To further explore its basis for clinical use, intra and interrater reliability of the NCAPC was investigated. Data collection was done by videotaping the participants before and during influenza vaccination. Intrarater reliability was evaluated by the first author on a group of 50 randomly selected individuals (mean age 42.5, range 19-72) and was found at 0.94. Interrater reliability was investigated in two stages. In the initial step different groups of health care workers (caregivers, nurses, case managers, and therapists), each including five raters, viewed a sample of 12 adult participants with IDD (3 at each level of IDD mean age was 49 years, range 16-72), that were extracted from the population sample. Interrater reliability of all raters within the groups varied from low to very high (ICC(1,1) = 0.40-0.88). Interrater reliability was very high in caregivers. The Physical- and Occupational therapists are one group were considered potential users of the measure. In the second stage 3 participants from each of the groups showing high interrater reliability (caregivers and therapist) evaluated interrater reliability in a randomly selected group of 40 individuals (mean age 41.2, range 15-72). Interrarter reliability for the therapists and caregivers was found at 0.91 and 0.92 correspondingly. The researchers conclude that that the NCAPC have been found to hold high reliability values. (Contains 2 figures and 6 tables.)