Recent evidence suggests that antiangiogenic therapy is sensitive to p53 status in tumors, implicating a role for p53 in the regulation of angiogenesis. Here we show that p53 transcriptionally activates the alpha(II) collagen prolyl-4-hydroxylase [alpha(II)PH] gene, resulting in the extracellular release of antiangiogenic fragments of collagen type 4 and 18. Conditioned media from cells ectopically expressing either p53 or alpha(II)PH selectively inhibited growth of primary human endothelial cells. When expressed intracellularly or exogenously delivered, alpha(II)PH significantly inhibited tumor growth in mice. Our results reveal a genetic and biochemical linkage between the p53 tumor suppressor pathway and the synthesis of antiangiogenic collagen fragments.