It has been proposed that stepwise progression occurs from atypical adenomatous hyperplasia (AAH) through bronchioloalveolar carcinoma (BAC) to invasive lung adenocarcinoma. However, the underlying molecular mechanisms have not been identified. We report a patient with a mixed adenocarcinoma of the lung that had different EGFR mutations in the papillary subtype, the acinar subtype, and the surrounding AAH and BAC areas. EGFR mutations may accumulate during tumor progression and lead to heterogeneity of EGFR mutations within the tumor.