Mizoribine is a new immunosuppressive drug and was authorized by the Japanese Government in 1984. The strong immunosuppressive activity of mizoribine was already demonstrated in various animal models, in renal transplantation and in steroid-resistant nephrotic syndrome. Recently, the remarkable clinical advantages of an imidazole for adults with rheumatoid arthritis, lupus nephritis and other rheumatic diseases were reported. Mizoribine is an imidazole nucleoside and the metabolites, MZ-5-P, exerts its activity through selective inhibition of inosine monophosphate synthetase and guanosine monophosphate synthetase, resulting in the complete inhibition of guanine nucleotide synthesis without incorporation into nucleotides. Thus, mizoribine is superior to azathioprine, in that it may not cause damages to normal cells and normal nucleic acid.