A precursor lesion of pulmonary adenocarcinoma has not been clearly defined. Previous studies suggested that atypical alveolar cell hyperplasia (AACH) might represent such a precursor lesion. Most previous studies showed an association between AACH and adenocarcinoma in surgical resection specimens. In this study, we searched for the prevalence of AACH and nonatypical alveolar cell hyperplasia (ACH) in a general autopsy population. Cases in which there was clinical or anatomic evidence of pulmonary neoplasia were excluded from the study. In the 100 consecutive autopsies examined, we found four cases of ACH and two cases of AACH. The two AACH lesions showed cytologic atypia and stained positively for p53 and c-erb-2. These findings suggest a possible role for AACH as a precursor lesion of adenocarcinoma of the lung.