Y‐box binding protein‐1 (YB‐1) is a member of the cold shock protein family and functions in transcription and translation. Many reports indicate that YB‐1 is highly expressed in tumor cells and is a marker for tumor aggressiveness and clinical prognosis. Here, we show clear evidence that YB‐1 is expressed in the angiogenic endothelial cells of various tumors, such as glioblastoma, esophageal cancer, gastric cancer, colon cancer, and lung cancer, as well as in tumor cells. YB‐1 was highly expressed in glomeruloid microvascular endothelial cells of brain tumors and microvessels in the desmoplastic region around multiple solid tumors. On the other hand, no or low YB‐1 expression was observed in normal angiogenic endothelial cells from fetal kidney, newborn lung, and placenta. The endothelial cells in inflammatory regions of granulomas were also weakly labeled. Knockdown of YB‐1 expression by small‐interfering RNA induced G1 cell cycle arrest and inhibited the growth of human umbilical vein endothelial cells stimulated by growth factors. Taken together, YB‐1 plays an important role in the growth of not only tumor cells but also tumor‐associated endothelial cells, suggesting that YB‐1 is a promising target for cancer therapy. (Cancer Sci 2010)