OBJECTIVE To investigate which maternal HLA allele or haplotype is primarily associated with isolated congenital complete heart block (CCHB) in offspring. METHODS HLA class II typings were assessed by line probe assay and polymerase chain reaction-sequence-specific oligonucleotide probe methods, and HLA class I by the microlymphocytotoxicity test, in 13 Italian anti-Ro-positive mothers of children with CCHB and 41 anti-Ro-positive mothers with healthy children (20 mothers with systemic lupus erythematosus [SLE] and 21 with Sjögren's syndrome [SS]). Anti-Ro antibodies were studied by immunoblot. RESULTS HLA-DRB1*03011 and DRB1*03011; DQA1*0501;DQB1*0201 were more frequent in mothers of infants with CCHB than in mothers who had SLE, but not in mothers who had SS and whose children were healthy. Mothers of infants with CCHB were either HLA-B5/35, B17, or B44 positive and had a higher prevalence of B44;DRB11;DQA1*0501;DQB1*0301 and isolated anti-52-kd antibodies, which were absent in SS and SLE controls. CONCLUSIONS Mothers of infants with CCHB presented a strong genetic similarity to mothers who had SS, except for HLA class I phenotype. HLA-DRB1*03011;DQA1*0501;DQB1*0201 seemed not to be primary CCHB-associated genes, but were involved in an SS-like anti-Ro/La response. The combined presence of HLA-DRB1*03011 and anti-52-kd SSA/Ro antibodies conveyed the highest risk of giving birth to an affected child.