Abnormalities of the respiratory function are a common finding in adult obesity. In order to investigate the occurrence of similar facts in pediatric age, a group of 39 obese children (20 girls and 19 boys, aged from 7 to 15 years) whose weight excess for their height ranged from 25 to 105 p. 100 was studied and compared to a control group of normal children of similar ages. Lung volumes, blood gases, transfer factor of the lung for CO, dynamic lung compliance, total lung resistance and ventilatory response to CO2 have been studied. In these obese children by contrast to obese adults, the vital capacity and the residual volume were normal. The blood gases, the transfer factor for CO, the dynamic lung compliance and the total resistances of the lung were similar to those of the normal group. The respiratory patterns were normal in all children but one who had during a short time a periodic ventilation with short periods of breath-holding. The ventilatory response to CO2 of the obese children was decreased and a highly significant correlation was found between the individual values of the respiratory response to CO2 and the percentage of weight excess. Different hypotheses are discussed to explain these results.