We have previously shown that recognition of cellular antigens presented by CD1d stimulates human invariant natural killer T (iNKT) cells to produce GM-CSF, which is a key hematopoietic cytokine. Here we show that IL-1β, IL-6, and PGE₂ are also produced during iNKT interactions with human monocytes. Since these factors play important roles during hematopoiesis, we used immunodeficient NSG mice to investigate the impact of iNKT cells on human umbilical cord blood engraftment in vivo. Human iNKT cells transferred into unconditioned NSG mice were detectable in bone marrow for up to 3 weeks, but did not expand or persist long-term. Transfer of total cord blood mononuclear cells (CBMC) alone resulted in persistence of human cells for several months but a nearly complete failure to develop B lymphocyte and myelomonocytic lineages. In contrast, co-transfer of CBMCs with iNKT cells led to the emergence of a prominent B cell population after 2 months and evidence of myelomonocytic cells after 3 months. Additionally, co-transfer of iNKT cells with suboptimal doses of CBMCs facilitated lasting human cell engraftment. Thus, the presence of iNKT cells during the initial period after transfer not only enhanced overall graft survival, but promoted the later emergence of diverse human hematopoietic lineages in this model system. These effects could be due to early imprinting of HSCs by factors produced by iNKT cells, or to a role for iNKT cells in creating a bone marrow microenvironment ("niche") that promotes HSC engraftment and differentiation. Determination of the cellular and molecular pathways by which iNKT cells influence human cord blood engraftment will allow for their use as a cellular immunotherapy to improve clinical outcomes of HSCT.