The interleukin-6 (IL-6) cytokine family plays an important role in regulating cellular responses during hematopoiesis. We report here that mice homozygous for a knock-in mutation in the IL-6 cytokine family receptor signaling subunit glycoprotein (gp) 130 (gp130Y757F/Y757F) that leads to gp130-dependent signal transducers and activators of transcription (STAT) 1/3 hyperactivation develop a broad spectrum of hematopoietic abnormalities, including splenomegaly, lymphadenopathy, and thrombocytosis. To determine whether STAT3 hyperactivation was responsible for the perturbed hematopoiesis in gp130Y757F/Y757F mice, we generated gp130Y757F/Y757F mice on a Stat3 heterozygous (Stat3+/-) background to specifically reduce gp130-dependent activation of STAT3, but not STAT1. Normal hematopoiesis was observed in gp130Y757F/Y757F:Stat3+/- bone marrow and spleen, with no evidence of the splenomegaly and thrombocytosis displayed by gp130Y757F/Y757F mice. The perturbed cellular composition of thymus and lymph nodes in gp130Y757F/Y757F mice was also alleviated in gp130Y757F/Y757F: Stat3+/- mice. Furthermore, we show that hematopoietic cells from gp130Y757F/Y757F mice exhibited increased survival and proliferation in response to IL-6 family cytokines. Collectively, these data provide genetic evidence that gp130-dependent STAT3 hyperactivation during hematopoiesis has pathological consequences affecting multiple organs, and therefore identify the threshold of STAT3 signaling elicited by IL-6 family cytokines as a critical determinant for hematopoietic homeostasis.