We hypothesize that aldehyde dehydrogenase (ALDH) isozymes may be upregulated in lung tissue as a result of exposure to carcinogenic aldehydes found in cigarette smoke. To investigate this hypothesis, we studied the expression of two ALDH isozymes in lung cancer from patient samples and its relationship to the history of cigarette smoking. Immunohistochemical staining for ALDH1A1 and ALDH3A1 was performed on archival specimens from control patients without lung cancer, and patients with one of the primary lung cancers: squamous cell cancer (SCCA), adenocarcinoma (AdenoCA), and small cell lung cancer (SCLC). An overall score was obtained for each sample based upon multiplying the staining intensity (0-3) and the extensiveness (0-100%). Mean+/-S.E.M. for each experimental group was calculated and compared. Our results indicate a significantly higher level of expression of ALDH1A1 and ALDH3A1 in SCCA (155+/-19 and 162+/-17, respectively) and AdenoCA (116+/-12 and 107+/-10) than SCLC (39+/-11 and 42+/-12) (P<0.01). Atypical pneumocytes demonstrated significantly higher levels of expression of ALDH1A1 and ALDH3A1 than normal pneumocytes (a normal counterpart of AdenoCA), which is suggestive of up regulation during malignant transformation to AdenoCA. A subset analysis of all samples studied revealed increased expression of ALDH1A1 (P=0.055) and ALDH3A1 (P=0.0093) in normal pneumocytes of smokers (n=32) in comparison to those of non-smokers (n=17). Non-small cell lung cancer (NSCLC) express very high levels of ALDH1A1 and ALDH3A1 in comparison with SCLC, elevated expression of both enzymes may be associated with malignant transformation to AdenoCA, and cigarette smoking seems to result in increased expression of these enzymes in normal pneumocytes.