Tumor-induced angiogenesis (TIA), i.e., the ability of transformed cells to stimulate new blood vessel formation is an important factor contributing to tumor growth and invasiveness. The antiangiogenic effect of the retinoids, all-trans retinoic acid, 13-cis retinoic and 9-cis retinoic acid, of 1,25-dihydroxyvitamin D3, and of their combinations were studied using an experimental system in vivo. TIA was induced in immunosuppressed mice by intradermal injection of the two human transformed keratinocyte lines, Skv-e2, harboring DNA of human papillomavirus (HPV) type 16, and HeLa, harboring HPV18 DNA. The three retinoids and 1,25-dihydroxyvitamin D3, when administered systemically to mice, before the angiogenesis assay significantly decreased TIA. Their combination led to a synergistic inhibition of TIA. These results provide the basis for the use of combination of retinoids and 1,25-dihydroxyvitamin D3 in treatment of neoplastic diseases.