Insulin-like growth factor II is a fetal promoter of cell proliferation that is involved in some forms of cancer and overgrowth syndromes in humans. Here, we provide two sources of genetic evidence for a novel, pivotal role of locally produced insulin-like growth factor II in the development of atherosclerosis. First, we show that homozygosity for a disrupted insulin-like growth factor II allele in mice lacking apolipoprotein E, a widely used animal model of atherosclerosis, results in aortic lesions that are approximately 80% smaller and contain approximately 50% less proliferating cells compared with mice lacking only apolipoprotein E. Second, targeted expression of an insulin-like growth factor II transgene in smooth muscle cells, but not the mere elevation of circulating levels of the peptide, causes per se aortic focal intimal thickenings. The insulin-like growth factor II transgenics presented here are the first viable mutant mice spontaneously developing intimal masses. These observations provide the first direct evidence for an atherogenic activity of insulin-like growth factor II in vivo.