The present invention relates to the generation of vascularized human heart tissue from human primordial Islet1-positive (ISL1+) progenitors, and more particularly the generation of vascularized human heart tissue from human primordial Islet1+ cardiovascular stem cells which are positive for markers ISL1+/NKX2.5−/KDR−. One aspect of the invention relates to isolation of human ISL1+ primordial cells from human pluripotent cells, such as human ES cells or other human pluripotent stem cell sources, wherein the human ISL1+ primordial cells can differentiate into three different lineages; cardiomyocyte lineages, endothelial lineages and smooth muscle lineages. Another aspect relates to use and implantation of the human primordial ISL1+ progenitors into an animal model to generate human vascularized heart tissue, and more particularly, the production of an in vivo humanized model of vascular disease. One embodiment relates to the use of an in vivo humanized model of vascular disease as an assay, for example to assess drug toxicity and/or identify agents which increase and decrease coronary blood flow to the human vascularized heart tissue. Another embodiment relates to the therapeutic use of human primordial ISL1+ progenitors, for example, in one embodiment the invention provides methods for the treatment cardiovascular disorders and/or congenital heart disease in a subject comprising transplanting into subjects vascularized human heart tissue generated from human ISL1+ progenitors.