MicroRNAs can govern up to hundred different mRNAs and are important regulators of gene expression programs in development and disease. We analyzed the expression of microRNA-21, one of the most common oncomirs, in non-small cell lung cancer (NSCLC). Using northern blots the microRNA-21 expression levels of NSCLC-derived tissue and cell lines were measured. In line with earlier observations we show that mature microRNA-21 expression levels are highly increased in NSCLC-derived tissue compared to normal lung tissue. Additionally, we demonstrate that microRNA-21 levels correlate with malignancy since its expression in higher staged tumors is significantly more elevated compared to stage 1A. Interestingly, microRNA-21 levels in cultured NSCLC-derived cells are comparable to the expression detected in non-malignant lung tissue. Since microRNA-21 levels showed no fluctuation during the cell cycle, accelerated proliferation of tumor cells is not responsible for microRNA-21 upregulation in the tumor compartment. Similarly to NSCLC-derived cancer cells, the tumor-associated fibroblasts show low expression levels of microRNA-21. Together, these data indicate that rather microenviromental and growth conditional changes than intrinsic features of the cancer cells are responsible for the observed increase of microRNA-21 levels in tumor tissues. Subsequently culturing conditions were changed to assess the impact of co-cultivation with fibroblasts, hypoxia and anchorage-independent growth on microRNA-21 expression. While co-cultivation with tumor-associated fibroblasts had no effect on microRNA-21 expression, both hypoxia and anchorage-independent growth cause a microRNA-21 elevation. In summary, our data demonstrate that growth conditions especially expected in more malignant tumors result in microRNA-21 upregulation explaining the observed increase in higher staged lung cancer tissue, but not in lung cancer-derived cells.