Failure to thrive is a common complication of childhood obstructive sleep apnea syndrome (OSAS). To further evaluate its cause, we obtained 3-day dietary records, anthropometric measurements, polysomnography, and measurements of energy expenditure during sleep (SEE) in children with OSAS before and after tonsillectomy and adenoidectomy. Fourteen children were studied (mean age, 4 +/- 1 (SD) years). During initial polysomnography, patients had 6 +/- 3 episodes of obstructive apnea/hr, an arterial oxygen saturation nadir of 85% +/- 8%, and peak end-tidal carbon dioxide tension of 52 +/- 6 mm Hg. After surgery, OSAS resolved in all patients. The standard deviation score (z score) for weight increased from -0.30 +/- 1.47 to 0.04 +/- 1.34 (p < 0.005), despite unaltered caloric intake (91 +/- 30 vs 90 +/- 27 kcal/kg per day; not significant). The initial SEE (averaged over all sleep states) was 51 +/- 6 kcal/kg per day; postoperatively, it decreased to 46 +/- 7 kcal/kg per day (p < 0.005). Although SEE decreased during all sleep stages, the greatest decrease occurred during rapid eye movement sleep. The patients with the highest SEE on initial study had the lowest z scores (r = -0.62; p < 0.05). We conclude that SEE decreases and weight improves after resolution of OSAS. We speculate that the poor growth seen in some children with OSAS is secondary to increased caloric expenditure caused by increased work of breathing during sleep.