The long-range apparent diffusion coefficient (LRADC) of ³He gas in lungs, measured over times of several seconds and distances of 1-3 cm, probes the connections between the airways. Previous work has shown the LRADC to be small in health and substantially elevated in emphysema, reflecting tissue destruction, which is known to create collateral pathways. We report here computer simulations and measurements of ³He gas diffusion in healthy lungs. The lung is generated with a random algorithm using well-defined rules, yielding a 3D set of nodes or junctions, each connected by airways to one parent node and two daughters, with airway dimensions taken from published values. Spin magnetization in the simulated lung is modulated sinusoidally and the diffusion equation is solved out to times of 1000 s. The modulated magnetization decays with a time constant corresponding to a LRADC of ~0.001 cm²/s, a factor of ~20 smaller than the values in healthy lungs measured here and previously, both in vivo and in explanted lungs. It appears that collateral gas pathways, not present in the simulations, are functional in healthy lungs; they provide additional and more direct routes for long-range motion than the canonical airway tree. This is surprising, as collateral ventilation is believed to be physiologically insignificant in healthy lungs. We discuss the effect on LRADC of small collateral connections through airway walls and rule out other possible mechanisms. The role of collateral paths is supported by measurements of smaller LRADC in pigs, where collateral ventilation is known to be smaller.