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To determine whether hyperpolarized helium-3 (HHe) diffusion MR can detect the expected enlargement of alveoli that occurs with lung growth during childhood.A total of 29 normal subjects aged four to 30 years underwent HHe diffusion MR imaging with the b-value pair 0, 1.6 second/cm2. A second acquisition during a separate breathhold was performed using the b-value pair 0, 4 second/cm2 to evaluate the dependence on b-value. The mean apparent diffusion coefficient (ADC) and lung volume for each acquisition and each subject was determined.Subjects as young as four years of age were able to cooperate with the imaging procedure. The mean ADC increased with increasing subject age (r = 0.8; P < 0.001), with a 55% increase in mean ADC from the youngest to oldest subject. Lung volumes measured on MR were highly repeatable for the two HHe MR acquisitions (r = 0.980, P < 0.001). The mean ADC values measured with the two different b-value pairs were highly correlated (r = 0.975; P < 0.001), but the higher b-value pair resulted in slightly lower mean ADCs (P < 0.001).HHe diffusion MR appears to detect the expected increase in alveolar size during childhood, and thus HHe MR may be a noninvasive method to assess development of the lung microstructure.