A technique of low-field pulsed proton nuclear magnetic resonance (NMR) spin relaxation is described for characterizing the porosity and (effective) pore size distribution in vitro in human compact bone. The technique involves spin–spin relaxation measurement and inversion spin–spin relaxation spectral analysis methods. The spin–spin relaxation decay curve is converted into a T₂ distribution spectrum by a sum of single exponential decays. The advantages of using low-field NMR for the spin–spin relaxation technique are illustrated. The results obtained from NMR methodology are compared with the results obtained from currently available but destructive histomorphometry and mercury porosimetry methods. The NMR porosities correlate well with the results obtained from the histomorphometry measurements of eight samples from donors of ages 21–89 years. The pore size distributions from T₂ relaxation measurements are similar to the distributions obtained from the mercury porosimetry and histomorphometry measurements. This indicates that the age-related porosity and pore size changes in human compact bone can be detected using the low-field NMR technique.