Neonatal lupus is a model of passively acquired autoimmunity in that immune abnormalities in the mother lead to the production of antibodies that cross the placenta and injure the developing fetus. Congenital complete heart block (CCHB), a permanent manifestation of neonatal lupus, is detectable after 18 wk gestation. Transient manifestations include cutaneous, hepatic, and hematologic abnormalities that occur at variable frequency. To date, there is a universal association of CCHB with maternal antibodies to SSA/Ro-SSB/La ribonucleoproteins, detectable by high ratio monomer:crosslinker SDS-immunoblot. Intriguingly, cardiac disease and often other manifestations are not present in the mother, raising the hypothesis that there is differential expression and/or accessibility of SSA/Ro-SSB/La antigens in fetal vs. adult tissues. CCHB may be a final consequence of a more widespread inflammatory response in the heart, including the existence of an associated myocarditis. In contrast to the in utero onset of CCHB, skin lesions generally become apparent after birth. Ultraviolet exposure may be an initiating factor and exacerbate an existing rash. Several studies have documented the predominance of DR3 alleles in mothers of affected offspring, frequently associated with the extended haplotype A1,B8. Available evidence suggests that fetal genetic differences in the major histocompatibility complex (MHC) do not influence susceptibility. The recommended clinical approach includes obstetric and rheumatologic management of both the fetus identified with CCHB and the fetus with a normal heart beat but at high risk of developing CCHB. Fetal echocardiogram is essential in diagnosing and following disease and may suggest the presence of an associated myocarditis.(ABSTRACT TRUNCATED AT 250 WORDS)