Using enzyme-linked immunosorbent assays, autoantibodies to Ro (SS-A) were detected in the sera of 21% of the first-degree relatives and 11% of the second-degree relatives of anti-Ro-positive probands with systemic lupus erythematosus (SLE) or primary Sjögren's syndrome, as compared with 3% of normal control subjects (P = 0.003 and P = 0.09, respectively). In a parallel study, anti-Ro occurred in 28% of the first-degree relatives of unselected members of families of SLE patients, regardless of the proband's serologic status, compared with 6% of the relatives from normal healthy families. Antibodies to La and to Sm/nuclear RNP were infrequent. Anti-Ro occurred in 41% of the relatives considered to have a dominant, non-HLA-linked "autoimmune trait" by virtue of having any autoimmune disorder and/or serologic abnormality (antinuclear antibodies, anti-single-stranded DNA, or biologic false-positive VDRL test result), as compared with only 2% of the healthy, seronegative relatives without the trait (P = 0.009). Moreover, HLA-DR2 and/or DR3 occurred in 90% of anti-Ro-positive subjects, regardless of their clinical status. These results demonstrate that the Ro autoantibody response occurs frequently in relatives of patients with SLE and is genetically mediated by both major histocompatibility complex and non-major histocompatibility complex effects.