Selectins have been shown to play an important role in angiogenesis. We have set out to synthesize multiple analogs of the selectin ligand sialyl Lewis X. We show here that analogs of sialyl Lewis X at concentrations of 50 μg/ml or greater significantly inhibited angiogenesis inin vitroassays of endothelial cell migration and proliferation (p < 0.05). These analogs also exerted significant inhibitory effects inin vivoangiogenesis assays using the chick chorioallantoic membrane at doses of 2 mg/chick or greater (p < 0.05). In contrast, the related carbohydrate Lewis X did not inhibit angiogenesis in bothin vitroandin vivoassays. These data indicate that angiogenesis can be inhibited specifically by synthetic analogs of sialyl Lewis X. These analogs may potentially be useful in the treatment of angiogenesis-related diseases.