Development of the walking ability and self-care of patients with Down syndrome is affected by their body weight determining their lifestyle to a great extent. Objectives: The study aimed at the determination of body mass index for persons living in residential institutions and families, exploration its impact on walking and self-care as two, objective factors of life quality. Method: Data collection of persons aged 3-35 with Down syndrome living in families covered seven counties, while those of living in residential institutions covered thirteen counties in Hungary. In the 183 cases studied 76 people in residential institutions, 107 people lived in families. The cross-sectional study was processed by non-random sample selection. The questionnaires were filled out by health visitors and care takers edited by their own. Results: 50.6% of adults and 26.1% of children belonged to the overweight or obese category. Their residence showed a significant correlation with the body mass index (p< 0.001). Overweight and obese persons in families, while thin ones were more prevalent in institutions. Regarding the walking ability and self-care of the persons living in families a significantly higher level of development was achieved (p< 0.001). Walking ability (p = 0.001) and self-care (p = 0,008) were worsened by less body weight significantly, while overweight or obesity influenced it less negatively. Discussion: The claim is not further acceptable whereas persons with Down syndrome are more prone to obesity than average people. However unfavourable weight gain in adults draws attention to the necessity to a healthy diet and regular exercise. The people living in residential institutions with significantly lower body mass index and the associated low development of walking ability and self-care envisages an urgent reform of residential institutions. Life in the institutions negatively affects the walking ability and self-care, and thus significantly reduces the quality of life of persons with Down's syndrome.