Isolated failure to thrive in an infant caused by chronic hypoventilation due to hypertrophic adenoids and tonsils, has not been previously described. A 9 month-old infant presented with weight loss and mild clinical respiratory signs. Hypoxemia and CO2 retention, however, were documented by transcutaneous gas measurement, and ENT examination revealed enlarged tonsils and adenoids. Adenoidectomy and tonsillectomy at 9 1/2 months of age completely reversed the patient's hypoventilation and growth pattern. While the exact mechanism of failure to thrive secondary to upper airway obstruction in infancy is not clear, we conclude that routine ENT evaluation should be a regular part of the workup of infants under one year who suffer from failure to thrive.