Revision with unchanged content. COPD and asthma have significant morbidity and mortality. Hyperpolarized (HP) 3He Diffusion MRI has been used to assess lung diseases. The results correlated well with histology, but the diffusion time for previous methods is limited to the order of milliseconds under 1.5 T. New MR methods were developed to measure HP 3He diffusion over the long-time scale (LTS: diffusion time on the order of seconds) in human lungs and were used to investigate COPD and asthma. The basic equations were developed, numerical simulation was performed, and measurements were made in phantoms. Measurements in healthy subjects confirmed that 3He diffusion still strongly depended on the diffusion time in LTS and the ADC decreased significantly. Reproducibility, anisotropy and dependence on parameters were characterized. Experiments in patients with COPD suggested that LTS diffusion was more sensitive to early COPD changes than short-time-scale diffusion. Measurements in patients with severe asthma revealed large regional elevations in LTS ADC maps while the changes in STS were smaller. And there was a good correlation between LTS ADC maps and areas of lowdensity "air-trapping" seen by CT.