Individuals with intellectual and developmental disabilities (IDD) are in need of effective and motivating physical fitness training programs. The aim was to test the effectiveness of a virtual reality (VR)-based exercise program in improving the physical fitness of adults with severe IDD when implemented by on-site caregivers. A research group (N = 20; mean age [plus or minus] standard deviation = 47.9 [plus or minus] 8.6 years; severe IDD level) was matched for age, IDD level and functional abilities with a comparison group (N = 24, mean age = 46.2 [plus or minus] 9.3 years; severe IDD level). An 8-week fitness program consisting of 2-3 30-min sessions per week included game-like exercises provided by the IREX/GX video capture VR system. Changes in physical fitness were monitored by changes in heart rate at rest. A significant (P less than 0.005) reduction in heart rate was demonstrated for the research group. No change in heart rate was monitored in the comparison group. Despite statistically significant improvements in heart rate, the results are not strong enough functionally to claim that this program improved physical fitness of individuals with severe intellectual disability. (Contains 2 tables and 1 figure.)