Physical fitness is an important marker for health. In this study we investigated the feasibility and reliability of health-related physical fitness tests in children with moderate to severe levels of intellectual disability. Thirty-nine children (2-18 yrs) performed tests for muscular strength and endurance, the modified 6-minute walk test (6mwt) for cardiorespiratory fitness, and body composition tests, and 30-97% of the tests were successfully completed. Short-term test-retest reliability of all tests was good (Intraclass Correlation Coefficient [ICC] > 0.8), long-term test-retest reliability was good for most tests (ICC > 0.7), but low ICCs were found for most strength tests. Measuring body composition and cardiorespiratory fitness is feasible and reliable. Measuring muscle endurance is fairly feasible and reliable.