Myeloproliferative syndromes (MPS) are a heterogeneous subclass of nonlymphoid hematopoietic neoplasms which are considered to be intrinsic to hematopoietic cells. The causes of MPS are largely unknown. Here, we demonstrate that mice deficient for retinoic acid receptor gamma (RARgamma), develop MPS induced solely by the RARgamma-deficient microenvironment. RARgamma(-/-) mice had significantly increased granulocyte/macrophage progenitors and granulocytes in bone marrow (BM), peripheral blood, and spleen. The MPS phenotype continued for the lifespan of the mice and was more pronounced in older mice. Unexpectedly, transplant studies revealed this disease was not intrinsic to the hematopoietic cells. BM from wild-type mice transplanted into mice with an RARgamma(-/-) microenvironment rapidly developed the MPS, which was partially caused by significantly elevated TNFalpha in RARgamma(-/-) mice. These data show that loss of RARgamma results in a nonhematopoietic cell-intrinsic MPS, revealing the capability of the microenvironment to be the sole cause of hematopoietic disorders.