Percentage body fat of 35 children with spina bifida and 129 age-matched normal children was measured by underwater weighing and skinfold thickness to assess obesity. Percentage body fat of patients below five years was similar to that of controls; however, 58 per cent of patients above six years had an increased percentage of body fat. The neurological level and ambulatory ability were associated with percentage body fat. A significant correlation between percentage body fat and hydrocephalus suggests that the metabolic and nutritional maladaptation is caused not only by these patients' physical inactivity but also by the condition itself. Appropriate nutritional and mobility programmes should be started early to prevent the development of obesity.