We studied 41 patients, 22 men and 19 women, with surgically resected adenocarcinoma (AD) of the lung, to determine the frequencies of associated alveolar cell hyperplasia (ACH) and atypical alveolar cell hyperplasia (AACH), and to define the immunohistochemical profiles (IHP) of the AD, ACH, and AACH. The criteria used to look for ACH included a single row of cuboidal cells along alveolar walls, cell morphology distinct from bronchiolar epithelium, and the absence of chronic inflammation. AACH was considered whenever nuclear size was double the size of neighboring ACH and/or in cases with marked nuclear irregularity or hyperchromatism. ACH was identified in 24 of the 41 cases of AD. AACH was further demonstrated in six of the 24 cases of ACH. Twenty-three of the 24 cases of ACH were suitable for IHP. The tumors yielded positive results in 23/23, 22/23, 17/23, 22/23, and 18/23 cases when stained with Cam 5.2, AE1/AE3, Leu M-1, CEA, and B72.3, respectively. ACH reacted positively in 17/23, 17/23, 0/23, 4/22, and 1/23 cases stained with Cam 5.2, AE1/AE3, Leu M-1, CEA, and B72.3, respectively. AACH reacted positively in 6/6, 6/6, 0/6, 1/6, and 0/6 cases stained with Cam 5.2, AE1/AE3, Leu M-1, CEA, and B72.3, respectively. These findings suggest that ACH and AACH are common features found in 24 and six of the 41 cases of AD, respectively. The significance of this finding is not known, but it is possible that ACH/AACH may represent precursor changes similar to the bronchial epithelial dysplasia described in squamous cell carcinoma of the lung.(ABSTRACT TRUNCATED AT 250 WORDS)