In order to develop a reliable and sensitive method for studying the development and progression of pulmonary emphysema, we compared stereological indices with the usual index for grade of emphysema, i.e., the mean linear intercept (Lm), in elastase-induced emphysema in mice. The Lm and stereological indices, including volumes of total lung tissue (V(lt)), airspaces ( V , , , ) , and surface area of alveolar walls were determined in B-p.m, H&E-stained, parafinembedded lung sections from elastase-(n = 7) or saline-treated (n = 8) mice. The indices were measured by point counting, using Cavalieri's principle (V(lt) and V(air)) or by counting intersections of alveolar walls with test lines of a known length (S,,, and Lm). Elastase treatment resulted in a significant increase of Lm and of V(air), both indicating airspace enlargement, and in a significant decrease of V(lt) and S(alv), indicating destruction of alveolar walls. Between each of the stereological indices and the Lm, significant correlations were found when all lungs were included, but not when the emphysematous lungs were considered separately. We conclude that stereological methods can be powerful morphometric tools for studying pulmonary emphysema development and progression, since they give information not only about the grade of airspace enlargement but also about the grade of destruction of alveolar walls. Based on this unique property, stereological methods also allow a distinction between pulmonary emphysema and unrelated conditions with dilatation of airspaces only.