It has been proposed that reduced let-7 expression causes RAS expression and correlates with poor survival of lung cancer cases, but little is known about correlations with clinicopathologic features. In this study, we examined 15 early bronchioloalveolar carcinomas (BACs), usually considered as adenocarcinomas in situ, as well as 26 well-differentiated and 25 less-differentiated invasive adenocarcinomas, to assess the association between tumor progression and let-7 expression levels. Additionally, we investigated 47 invasive lung adenocarcinomas for EGFR and KRAS mutations and correlations with let-7 levels. Relative to the corresponding normal lung tissue, reduced let-7 expression was observed in 13 of 15 BACs (87%) and totally in 52 of the 66 adenocarcinomas (79%), suggesting a link with early occurrence in carcinogenesis. On classification of adenocarcinomas into two groups according to let-7 expression, no prognostic or genetic differences were observed. Interestingly, some differences between histological subtypes were observed, such as lower let-7 expression levels in acinar adenocarcinomas and mucinous BACs.