PURPOSE:: To describe the demographics, medical diagnoses, and initial aerobic fitness levels of children participating in Fitkids: an exercise therapy program for children with chronic conditions or disabilities in the Netherlands. METHODS:: We reviewed data of children who were in the program on September 2010. RESULTS:: In total, 2482 children from 105 Fitkids centers were included. Results showed the large heterogeneity of the population regarding demographic characteristics and medical diagnoses. Significantly reduced scores on the 6-minute walk test and half Bruce treadmill test were observed. CONCLUSION:: The Fitkids population has great heterogeneity. In addition, a plethora of fitness tests were used, and registration of data in the Fitkids database was suboptimal. Moreover, this study showed the impaired aerobic fitness of children participating in Fitkids. Future research should investigate the effectiveness of the Fitkids program.