In adults with unilateral lung disease, pulmonary gas exchange is better when the patients is positioned with the good lung dependent. We studied the effects of body position on gas exchange in 10 infants with unilateral lung disease by measuring transcutaneous oxygen and carbon dioxide pressures in the supine and right and left lateral positions. We also performed krypton lung scans and measured changes in thoracic gas volumes in four of the infants. Transcutaneous oxygen pressure (mean +/- S.E.) was greater with the good lung uppermost (82 +/- 7.6 mm Hg) than with the good lung dependent (73 +/- 7 mm Hg) (P less than 0.02) or in the supine position (78 +/- 7 mm Hg). There were no changes in transcutaneous carbon dioxide pressure. The proportion of ventilation to the good lung was greater with the good lung uppermost than with the good lung dependent (P less than 0.01) or in the supine position (P less than 0.02) (64 +/- 3, 46 +/- 6, and 59 +/- 7 per cent, respectively). There were no significant changes with position in functional residual capacity, tidal volume, or dynamic lung compliance. We conclude that oxygenation in infants with unilateral lung disease is best with the good lung uppermost--the reverse of what has been observed in adults.