Individuals with intellectual and developmental disabilities (IDD) are in need of effective 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 IDD. A research group (N = 30; mean age = 52.3 plus or minus 5.8 years; moderate IDD level) was matched for age, IDD level and functional abilities with a control group (N = 30, mean age = 54.3 plus or minus 5.4 years). A 5-6 week fitness program consisting of two 30 min sessions per week included game-like exercises provided by the Sony PlayStation II EyeToy VR system. Changes in physical fitness were monitored by the Energy Expenditure Index (EEI), the modified 12 min walk/run and the Total Heart Beat Index (THBI). Significant (p less than 0.05) improvements in physical fitness were demonstrated for the research group in comparison to the control group for the Modified Cooper test and the THBI but not for the EEI test. The EEI, Modified Cooper and THBI tests were found feasible to evaluate physical fitness levels and change of individuals with IDD under clinical conditions. VR technology intervention was suitable for adults with IDD and resulted in significant improvements in the physical fitness levels of the participants. (Contains 3 tables and 3 figures.)