To determine whether factors other than disease activity influence immunoglobulin levels in patients with systemic lupus erythematosus (SLE), the effect of age, sex, race, and duration of disease on serum IgG and IgM levels in 170 patients with SLE were investigated. Serum IgM and IgG levels did not differ between men and women, while IgM levels were higher in whites. Serum IgG levels did not vary with age or duration of SLE. In contrast, serum IgM levels were negatively correlated with both age (r = -0.236; p = 0.002) and duration of SLE (r = 0.248; p = 0.001), and demonstrated a U-shaped age relationship, being higher in children and older patients. These patterns of immunoglobulin expression in patients with SLE contrast with those exhibited in populations of healthy individuals, suggesting that the immunoregulatory disturbances of SLE predominate over the normal mechanisms regulating levels of IgM and IgG.