Estrogenic hormones possess both immunostimulating and immunosuppressive properties. In systemic lupus erythematosus (SLE), pregnancy is associated with disease flares. In some situations, exogenous estrogen predisposes to development of new SLE, flares of preexisting SLE, and thromboses in susceptible individuals. In contrast, treatment with exogenous estrogen protects postmenopausal rheumatoid arthritis (RA) patients from active RA and osteoporosis. The search for estrogen-like compounds with anti-inflammatory properties may expand treatment options in RA.