RATIONALE: The current hypothesis that human pulmonary alveolarization is complete by 3 years is contradicted by new evidence of alveolarization throughout adolescence in mammals. OBJECTIVES: We re-examined the current hypothesis using helium-3 ((3)He) magnetic resonance (MR) to assess alveolar size non-invasively between 7-21 years, during which lung volume nearly quadruples. If new alveolarization does not occur, alveolar size should increase to