Formation and remodeling of blood vessels during development and disease involves a precisely regulated network of attractants and repellants that is similar across species. Various signaling pathways control the behavior of endothelial cells—the cells that line the interior surface of blood vessels. The objective of this study is to identify microRNAs—small, non-coding RNAs that help control gene expression—that contribute to the regulation of new blood vessel formation. We show that a microRNA called miR-10 regulates the behavior of endothelial cells during blood vessel formation by encouraging signals that promote blood vessel growth. Genetic experiments in zebrafish revealed that miR-10 functions, in part, by directly regulating the level of a protein called FLT1, which inhibits the behavior of endothelial cells that promotes new blood vessel growth. The ability to regulate blood vessel growth is important for treating many diseases, including cancer.