Acinar development in infant humans has not been extensively studied. Hyperpolarized gas diffusion MRI has been shown to relate directly to alveolar-airspace size in adults, pediatrics, canines, and mice. Using ex-vivo lungs from 7 healthy and 1 diseased infant humans, we investigated the relationship between 3He apparent diffusion coefficient (ADC) via mono-exponential decay, alveolar-duct radius via a restricted diffusion model originally developed for mice, and radius via histological measurement. While the mouse model is invalid in the infant diffusion regime, ADC measurements reflect changes in alveolar-airspace size. This method shows promise for longitudinal in-vivo acinar-airway monitoring in neonatal patients.