In this study we compared a variety of indexes of ventilation inhomogeneity based on multibreath nitrogen washout of the lungs. We studied 40 subjects in 5 clinical groups: 6 normal subjects, 5 "normal" smokers, 6 subjects with asthma, 13 with diffuse interstitial lung disease (DILD), and 10 with chronic obstructive pulmonary disease (COPD). We found that a moment ratio (micron 1/micron 0), which is a mean dilution number (or number of volume turnovers), is superior to other indexes studied with respect to minimal intrasubject variability and maximal diagnostic sensitivity. Even minimal ventilation inhomogeneity could be detected in "normal" smokers, as evidenced by the difference in micron 1/micron 0 values from those found in normal nonsmokers. As a group, asthmatics showed mild ventilation inhomogeneity, with greater variation depending on their functional status at the time of testing. Subjects with DILD tended to have moderate ventilation inhomogeneity, which appeared to increase with age. As expected, the greatest ventilation inhomogeneity occurred in COPD.