Smooth-muscle specimens from the lower esophagus of nine patients operated on for esophageal achalasia were examined with routine hematoxylin-eosin staining. This procedure revealed only a few eosinophils in or between the external smooth-muscle layers. Using specific immunohistochemical methods for the detection of the eosinophil cationic protein (ECP), however, varying degrees of eosinophil infiltration and extracellular deposit of ECP were disclosed in the achalasia specimens. The ECP also reacted with the monoclonal antibody, EG2, indicating secretion of the cytotoxic ECP. Few or no eosinophils were seen in the muscularis externa in specimens from six control patients without esophageal disease. In two controls many eosinophils were observed in the muscularis externa. However, no extracellular ECP was detected and very few eosinophils reacted with the monoclonal antibody (EG2), suggesting that these eosinophils were not activated. Depletion or total absence of peptidergic innervation was seen in all achalasia specimens but not in controls. Since the eosinophil cationic protein (ECP), in its activated form, is cytotoxic, we propose a pathogenic role of the eosinophil infiltration in achalasia.