Mean nuclear areas (MNA) were calculated morphometrically in ten cases of well-differentiated adenocarcinoma, ten cases of atypical alveolar cuboidal cell hyperplasia (AAH), and five cases of alveolar cuboidal cell hyperplasia (AH) and were compared with each other. In cases of adenocarcinoma, the MNA were significantly larger than those of AAH and AH, and the standard deviation (SD) of nuclear area was greater in adenocarcinoma than that in AAH and AH, thus implying greater "scatter" of the nuclear areas in the former than the latter two. The MNA and the SD of nuclear area in AH were smallest in these three groups. In one case each of AAH and adenocarcinoma, there were two different populations of nuclear size in the same lesions, that is, histograms showed both adenocarcinoma and AAH in the same tumor. Through the use of the morphometric method, many cases of AAH were easily distinguishable from adenocarcinoma cases, and there were two cases in which foci of adenocarcinoma and AAH coexisted.