RARA (retinoic acid receptor alpha) haploinsufficiency is an invariable consequence of t(15;17)(q22;q21) translocations in acute promyelocytic leukemia (APL). Retinoids and RARA activity have been implicated in hematopoietic self-renewal and neutrophil maturation. We and others therefore predicted that RARA haploinsufficiency would contribute to APL pathogenesis. To test this hypothesis, we crossed Rara^+/− mice with mice expressing PML (promyelocytic leukemia)-RARA from the cathepsin G locus (mCG-PR). We found that Rara haploinsufficiency cooperated with PML-RARA, but only modestly influenced the preleukemic and leukemic phenotype. Bone marrow from mCG-PR^+/− × Rara^+/− mice had decreased numbers of mature myeloid cells, increased ex vivo myeloid cell proliferation, and increased competitive advantage after transplantation. Rara haploinsufficiency did not alter mCG-PR–dependent leukemic latency or penetrance, but did influence the distribution of leukemic cells; leukemia in mCG-PR^+/− × Rara^+/− mice presented more commonly with low to normal white blood cell counts and with myeloid infiltration of lymph nodes. APL cells from these mice were responsive to all-trans retinoic acid and had virtually no differences in expression profiling compared with tumors arising in mCG-PR^+/− × Rara^+/+ mice. These data show that Rara haploinsufficiency (like Pml haploinsufficiency and RARA-PML) can cooperate with PML-RARA to influence the pathogenesis of APL in mice, but that PML-RARA is the t(15;17) disease-initiating mutation.