To determine the prevalence of autoimmune diseases and autoantibodies in relatives of Caucasian patients with systemic lupus erythematosus (SLE) we questioned 118 patients for the prevalence of autoimmune diseases in their relatives. Multicase SLE families were selected for further investigation: assessment of the presence of antinuclear antibodies (ANA), thyroid antibodies and IgM rheumatoid factor (IgM-RF). Thirty-three patients reported the presence of 50 autoimmune diseases in 43 relatives. Twenty-two diagnoses could be either confirmed (n=14) or refuted (n=8). SLE clustered significantly within families of SLE patients. Multiple sclerosis and rheumatoid arthritis also seemed to cluster within families of lupus patients. The prevalence of ANA (24%) and thyroid antibodies (44%) in 29 relatives of multicase SLE families was raised (P<0.05). In conclusion, the prevalence of autoimmune diseases is raised in relatives of Caucasian SLE patients. Also, the prevalence of autoantibodies is raised in relatives of multicase SLE families, both suggesting genetic influences in the pathogenesis of the disease. These findings support the genome-wide screening of SLE patients to unravel factors responsible for genetic susceptibility to SLE.