Repopulation of immunodeficient mice remains the primary method to assay human hematopoietic stem cells (HSCs). Here we report that female NOD/SCID/IL-2Rgc-null mice are far superior in detecting human HSCs (Lin−CD34+CD38−CD90+CD45RA−) compared with male recipients. When multiple HSCs were transplanted, female recipients displayed a trend (1.4-fold) toward higher levels of human chimerism (female vs male: injected femur, 44.4 ± 9.3 vs 32.2 ± 6.2; n = 12 females, n = 24 males; P = .1). Strikingly, this effect was dramatically amplified at limiting cell doses where female recipients had an approximately 11-fold higher chimerism from single HSCs (female vs male: injected femur, 8.1 ± 2.7 vs 0.7 ± 0.7; n = 28 females, n = 20 males; P < .001). Secondary transplantations from primary recipients indicate that females more efficiently support the self-renewal of human HSCs. Therefore, sex-associated factors play a pivotal role in the survival, proliferation, and self-renewal of human HSCs in the xenograft model, and recipient sex must be carefully monitored in the future design of experiments requiring human HSC assays.