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SMARCA4 is the catalytic subunit of the SWI/SNF chromatin-remodeling complex, which alters the interactions between DNA and histones and modifies the availability of the DNA for transcription. The latest deep sequencing of tumor genomes has reinforced the important and ubiquitous tumor suppressor role of the SWI/SNF complex in cancer. However, although SWI/SNF complex plays a key role in gene expression, the regulation of this complex itself is poorly understood. Significantly, an understanding of the regulation of SMARCA4 expression has gained in importance due to recent proposals incorporating it in therapeutic strategies that use synthetic lethal interactions between SMARCA4-MAX and SMARCA4-SMARCA2. In this report, we found that the loss of expression of SMARCA4 observed in some primary lung tumors, whose mechanism was largely unknown, can be explained, at least partially by the activity of microRNAs (miRNAs). We reveal that SMARCA4 expression is regulated by miR-101, miR-199 and especially miR-155 through their binding to two alternative 3'UTRs. Importantly, our experiments suggest that the oncogenic properties of miR-155 in lung cancer can be largely explained by its role inhibiting SMARCA4. This new discovered functional relationship could explain the poor prognosis displayed by patients that independently have high miR-155 and low SMARCA4 expression levels. In addition, these results could lead to application of incipient miRNA technology to the aforementioned synthetic lethal therapeutic strategies.