The Xe-133 ventilation studies of 115 patients were analyzed to determine the relative abilities of the single-breath and washout phases to detect regional ventilation abnormalities. All Xe-133 images were obtained in the posterior projection before 6-view perfusion studies with Tc-99m human albumin microspheres. There were 275 regions with matching V-P abnormalities in the patients. The washout portion of the study detected 258 of these regions (94%) and the single breath detected 175 (64%) (p less than 0.01). The discrepancies were confined to regions with nonsegmental perfusion defects, where the single breath detected 139 matches and the washout 216. The discrimination ratio between normal areas and areas of obstructive lung disease improved from 2 to 1 after 1 min washout to 30 to 1 after 5 min. The late phases of Xe-133 washout are useful in detecting ventilation abnormalities, especially those associated with nonsegmental perfusion defects.