MicroRNAs (miRNAs) are a broad class of small non-coding RNAs that control expression of complementary target messenger RNAs. Dysregulation of miRNAs has been described in various disease states including cancer and cardiac disease. A particular miRNA that was consistently reported to be upregulated in both cancer and various forms of cardiovascular diseases is miR-21. MiR-21 exerts oncogenic activity and therefore is considered as an oncomir. In the cardiovascular system miR-21 is enriched in fibroblasts and contributes to the development of fibrosis and heart failure. MiR-21 therefore emerges as an interesting candidate for the development of therapeutic strategies against many forms of cancer as well as heart diseases. Indeed, treatment with anti-miR-21 oligonucleotides reduced breast cancer growth. Inhibition of miR-21 by synthetic miRNA antagonists (antagomirs) improved heart function in a cardiac disease model. The same beneficial effects were observed in miR-21 knockout mice subjected to pressure-overload of the left ventricle underlining the key role of miR-21 as a therapeutic target. We here overview the current patent situation about the therapeutic use of miR-21 modulation in cancer and cardiovascular disease.