OBJECTIVE To assess the usefulness of enumerating argyrophilic nucleolar organizer regions (AgNORs) in assisting with the diagnosis, as well as its correlation with the degree of differentiation and DNA ploidy, in lung cancer. METHODS The specimens, taken from 151 benign and malignant pulmonary tumors, were stained with silver nitrate before the number of AgNORs was counted. DNA ploidy was determined in 58 cases of lung cancers. All cases of adenocarcinoma and squamous cell carcinoma were histologically classified as well, moderately or poorly differentiated. RESULTS Lung cancers had more AgNORs than benign tumors, and small cell carcinomas had fewer AgNORs than non-NSCLC (P < .001). The mean AgNOR number in adenocarcinoma increased progressively in accordance with poorer differentiation (P < .001). There was no correlation between the number of AgNORs and DNA ploidy. CONCLUSIONS Counting AgNORs helps with the differential diagnosis of lung cancers. In adenocarcinoma of the lung, the AgNOR number increases as the degree of differentiation decreases.