MicroRNA-155 (miR-155) is a critical regulator of immune cell development and function, but its mechanistic basis in the hematopoietic system remains unclear. This study identifies Src homology-2 domain-containing inositol 5-phosphatase 1 (SHIP1) as a direct target of miR-155. Using gain and loss of function approaches, the authors show that miR-155 represses SHIP1 through direct 3'UTR interactions, which are highly conserved across evolution. Overexpression of miR-155 in hematopoietic cells in vitro and in vivo leads to increased activation of the kinase Akt and reduced SHIP1 levels. Physiological regulation of miR-155 in macrophages also results in enhanced SHIP1 expression. In mice, specific knockdown of SHIP1 in the hematopoietic system using a miR-155-formatted siRNA recapitulates the myeloproliferative disorder observed in miR-155-expressing mice. These findings reveal a molecular link between miR-155 and SHIP1, suggesting that repression of SHIP1 is an important component of miR-155 biology and may contribute to human myeloproliferative disorders and myeloid leukemias.MicroRNA-155 (miR-155) is a critical regulator of immune cell development and function, but its mechanistic basis in the hematopoietic system remains unclear. This study identifies Src homology-2 domain-containing inositol 5-phosphatase 1 (SHIP1) as a direct target of miR-155. Using gain and loss of function approaches, the authors show that miR-155 represses SHIP1 through direct 3'UTR interactions, which are highly conserved across evolution. Overexpression of miR-155 in hematopoietic cells in vitro and in vivo leads to increased activation of the kinase Akt and reduced SHIP1 levels. Physiological regulation of miR-155 in macrophages also results in enhanced SHIP1 expression. In mice, specific knockdown of SHIP1 in the hematopoietic system using a miR-155-formatted siRNA recapitulates the myeloproliferative disorder observed in miR-155-expressing mice. These findings reveal a molecular link between miR-155 and SHIP1, suggesting that repression of SHIP1 is an important component of miR-155 biology and may contribute to human myeloproliferative disorders and myeloid leukemias.