Twenty-seven hypertensive patients (23 of whom were black) were treated with hydralazine as their major antihypertensive drug and were followed for evidence of autoimmunity and clinical systemic lupus erythematosus (SLE). Only one patient developed SLE but many, although asymptomatic, had serologic evidence of autoimmunity: antibodies to single- and double-stranded ribonucleic acid (RNA), single-stranded deoxyribonucleic acid (DNA), histones, and lymphocytes. Acetylation phenotype profoundly influenced this response; slow acetylators had a higher incidence and larger amounts of autoantibodies. Antibodies to both types of RNA were a more sensitive index of autoimmunity than antinuclear antibodies. Hydralazine treatment did not alter cell-mediated immune responses. The hydralazine SLE patient had large amounts of autoantibodies that were predominantly IgG, while in the others IgM autoantibodies were predominant. No antibodies, but positive lymphoproliferative responses to hydralazine, were found in half the patients tested.