Regional lung density (LD) and lung water (LW) measurements were made with positron transmission and emission tomography (PET) in normal and edematous lung in supine dogs in vivo. A comparison was also made of LD measurements by PET and X-ray CT (used by others to noninvasively assess pulmonary edema). Mean LW was 0.25 +/- 0.06 ml water/ml lung and the mean LD (PET) was 0.32 +/- 0.06 g/ml lung (average ratio of LW to LD was 0.795 +/- 0.041). The LD measurements ranged from 0.25 +/- 0.06 in anterior portions of lung to 0.43 +/- 0.11 g/ml in posterior areas, but the ratio of LW to LD was similar throughout the lung. The LW and LD measurements obtained in both normal and edematous portions of lung were strongly correlated (r = 0.886). Values for LD by PET were consistently higher than values obtained for LD by X-ray CT. These differences are probably due to beam-hardening effects with CT and partial-volume averaging and scattered radiation effects with PET. Nevertheless, PET-LD measurements may be a satisfactory method for following acute changes in LW or for normalizing other PET-derived data.