Chagas' disease, which is caused by the protozoan parasite Trypanosoma cruzi, is a significant public health problem in the Americas. Its clinical presentation varies significantly in different geographic regions. Using experimental infection in mice, we studied the pathologic changes in lungs in the acute phase of the disease caused by three Mexican isolates of T. cruzi. Clusters of parasites and inflammatory reactions were found in the walls of conducting airways and pulmonary vessels. Inflammation was more intense in the small vessels. Although the parasites were not found in the alveolar walls, severe pathologic changes in these structure were observed and included alveolar wall thickening and inflammatory infiltration. Furthermore, serous liquid, fibrin fibers, hyaline membranes, and erythrocytes were found in the alveolar spaces. The pathomorphologic changes observed in the infected mice are consistent with pneumonitis.