VEGF receptor 1 (VEGFR-1/Flt-1) is a high-affinity tyrosine kinase (TK) receptor for VEGF and regulates angiogenesis as well as monocyte/macrophage functions. We previously showed that the osteoclast deficiency in osteopetroticCsf1op/Csf1op(op/op) mice is gradually restored in an endogenous, VEGF-dependent manner. However, the molecular basis of the recovery is still not clear. To examine which VEGFR is important and to clarify how colony-stimulating factor 1 (CSF-1) and VEGF signals interact in osteoclastogenesis, we introduced a VEGFR-1 signaling deficiency (Flt1TK-/-) intoop/opmice. The originalFlt1TK-/-mice showed mild osteoclast reduction without bone marrow suppression. The double mutant (op/opFlt1TK-/-) mice, however, exhibited very severe osteoclast deficiency and did not have numbers of osteoclasts sufficient to form the bone marrow cavity. The narrow bone marrow cavity in theop/opFlt1TK-/-mice was gradually replaced with fibrous tissue, resulting in severe marrow hypoplasia and extramedullary hematopoiesis. In addition to osteoclasts, osteoblasts also decreased in number in theop/opFlt1TK-/-mice. These results strongly suggest that the interaction of signals by means of VEGFR-1 and the CSF-1 receptor plays a predominant role not only in osteoclastogenesis but also in the maintenance of bone marrow functions.