Caregivers of persons with profound intellectual and multiple disabilities (PIMD) often describe the quality of the daily movements of these persons in terms of flexibility or stiffness. Objective outcome measures for flexibility and stiffness are muscle tone or level of spasticity. Two instruments used to grade muscle tone and spasticity are the Modified Ashworth Scale (MAS) and the Modified Tardieu Scale (MTS). To date, however, no research has been performed to determine the psychometric properties of the MAS and MTS in persons with PIMD. Therefore, the purpose of this study was to determine the feasibility, test-retest reliability, and interrater reliability of the MAS and MTS in persons with PIMD. We assessed 35 participants on the MAS and MTS twice, first for the test and second a week later for the retest. Two observers performed the measurements. Feasibility was assessed based on the percentage of successful measurements. Test-retest and interrater reliability were determined by using the Wilcoxon signed rank test, intraclass correlation coefficients (ICC), Spearman's correlation, and either limits of agreement (LOA) or quadratically weighted kappa. The feasibility of the measurements was good, because an acceptable percentage of successful measurements were performed. MAS measurements had substantial to almost perfect quadratically weighted kappa (>0.8) and an acceptable ICC (>0.8) for both inter- and intrarater reliability. However, MTS measurements had insufficient ICCs, Spearman's correlations, and LOAs for both inter- and interrater reliability. Our data indicated that the feasibility of the MAS and MTS for measuring muscle tone in persons with PIMD was good. The MAS had sufficient test-retest and interrater reliability; however, the MTS had an insufficient test-retest and interrater reliability in persons with PIMD. Thus, the MAS may be a good method for evaluating the quality of daily movements in persons with PIMD. Providing test administrators with training and clear instructions will improve test reliability.