MRL/MpJ‐lpr/lpr mice spontaneously develop a lupus‐like autoimmune disorder characterized by massive proliferation of T cells and rapidly fatal immune complex glomerulonephritis. We evaluated the therapeutic effect of 5‐azacytidine (5AC), a cytidine analogue known as an inhibitor of DNA methylation, in MRL/MpJ‐lpr/lpr mice. Intraperitoneal injection of 5AC (50 μg, twice a week) starting from 6 weeks of age retarded the development of lymphadenopathy and autoimmune syndrome. Its beneficial effects included: (a) increased life‐span, (b) diminution of lymphadenopathy and splenomegaly, (c) reduction in circulating levels of autoantibodies such as anti‐DNA and rheumatoid factors, and (d) suppression of lupus glomerulonephritis. However, similar treatment in BALB/c mice did not affect the development of IgG anti‐human IgG antibody responses. These results suggest that the protective effect of 5AC is related to the inhibition of the lpr gene‐induced T cell proliferation, thereby suppressing the autoimmunity‐accelerating effect mediated by the lpr gene.