5-Hydroxy-2-(2,6-diisopropylphenyl)-1H-isoindole-1,3-dione (5HPP-33: 10), which was obtained during our previous structural development studies on thalidomide, was revealed to possess potent anti-angiogenic activity in a human umbilical vein endothelial cell (HUVEC) assay. Thalidomide (1) and its metabolite, 5-hydroxythalidomide (5-HT: 2), which possesses a hydroxyl group at the position corresponding to that of 5HPP-33, as well as IMiDs (immunomodulatory derivatives of thalidomide: 3 and 5), also showed weak or moderate activity in the same assay.