Neovascularization in the rheumatoid synovium plays an important role in the propagation of rheumatoid synovitis because the emigration of mononuclear cells and the growth of pannus are critically dependent on the development of small blood vessels. Inhibition of local vascular endothelial cell (EC) proliferation, which is essential for growth of these vessels, therefore, would have the potential to suppress rheumatoid inflammation. We investigated the effects of methotrexate (MTX), low doses of which are commonly administered to rheumatoid arthritis patients, on DNA synthesis by human umbilical vein EC in vitro and on rabbit corneal neovascularization in vivo. MTX inhibited both basal and EC growth factor-stimulated tritiated deoxyuridine (3H-UdR) incorporation into EC in a dose-dependent manner. Significant inhibition was observed at a concentration of 5 x 10(-9) M, which is that attained in the serum of treated patients. Neovascularization in vivo was also suppressed by low-dose intramuscular injections. These results suggest that MTX has an antiangiogenic effect, and may suppress rheumatoid inflammation through the reduction of synovial small blood vessels responsible for mononuclear cell infiltration and proliferation of synovial tissue.