The proportion of lung cancers classified as adenocarcinoma has steadily increased and now comprises nearly ½ of cases. However, the proportion of adenocarcinoma in situ is uncertain. Previous reports range from 5-10% in a large series to as high as 24% in the large Surveillance, Epidemiology and End Results database. Thus, these early adenocarcinoma lesions may represent a disproportionately large number of lung cancers which are PET negative yet carry an excellent prognosis after early resection. The early adenocarcinoma subtypes represent a clinical entity requiring further characterization to distinguish lesions more likely to be malignant from benign.