Eight normal human lungs obtained from patients dying from causes not related to the lung were subjected to morphometric analysis to determine the number of cells in the alveolar region and their mean volume and surface characteristics. The age range was 19 to 40 yr, average body weight was 74 kg, and the average fixed lung volume was 4,300 ml. The overall mean nuclear diameters of the nuclei of 5 major cell types in the lung parenchyma were found to have little variation, with means ranging from 7.54 to 8.77 micrometers. Alveolar type I epithelial cells were found to comprise 8% of the cells and to be one of the largest cells, having a mean volume of 1,764 micrometers 3 and covering an average of 5,098 micrometers 2 of alveolar surface. Seven percent of the alveolar surface was covered by alveolar type II cells, which make up 16% of the total alveolar cells and have a mean volume that is half that of the type I pneumocyte. Capillary endothelial cells make up 30% of the lung cells and were significantly smaller in both size and average surface area than the alveolar type I cells. Cells in the interstitial space comprised 37% of the total cells. The number of alveolar macrophages showed great variability, ranging from 19% of alveolar cells in 1 person to 3 to 5% in the nonsmoking females. The alveolar cell population characteristics found in resected lobes from 2 nonsmoking females were found to be similar to 2 nonsmoking females studied after autopsy. An interspecies comparison of characteristics of cells from the alveolar regions of normal lungs from humans, baboons, and rats showed that proportions of cells in the alveolar region and their average thickness, size, and surface areas were relatively constant.