Regional ventilation-perfusion ratios have been determined in 12 healthy subjects, 16 patients with pulmonary embolism, and 22 patients with chronic obstructive lung disease. The ventilation-perfusion ratios were determined from xenon-133 ventilation studies and 99Tcm-labeled particle perfusion scans, using either the fractional exchange of air or the relative distribution of tidal volume per unit volume as the numerator of the ratio. A comparison of these two methods showed comparable distributions of regional ventilation-perfusion relationships in the healthy subjects and patients with pulmonary embolism. However, in the patients with chronic obstructive pulmonary disease, the fractional exchange method clearly separated this group of patients from the others.