We have described previously the clinical features of a unique group of anti-Ro(SS-A) antibody positive Sjogren's patients who have cutaneous features of lupus erythematosus, most commonly subacute cutaneous lupus erythematosus, defined as the Sjogren's/lupus erythematosus overlap syndrome. Three of these patients are also mothers of infants with the neonatal lupus erythematosus syndrome, characterized by cutaneous lesions resembling subacute cutaneous lupus erythematosus or congenital heart block. Patients with Sjogren's/lupus erythematosus overlap syndrome, subacute cutaneous lupus erythematosus, and mothers of infants with the neonatal lupus syndrome characteristically have autoantibodies to Ro(SS-A), and in many cases, La(SS-B) antigens. The present study was designed to test the hypothesis that anti-Ro(SS-A)/La(SS-B) positive Sjogren's/lupus overlap patients and mothers of infants with neonatal lupus erythematosus syndrome are immunogenetically homogenous and closely related. We report a strong association with HLA-B8, DR3, DQw2, and DRw52 phenotypes and the HLA-B8, DR3, DQw2, DRw52 extended haplotype in both patient cohorts. Furthermore, we describe disease associations with HLA-DR3/DRw6 heterozygotes in both patient groups. These data demonstrate that anti-Ro(SS-A)/La(SS-B) positive Sjogren's/lupus overlap patients and neonatal lupus syndrome mothers are immunogenetically closely related to each other and appear to be more closely related to both primary Sjogren's syndrome and subacute lupus erythematosus, than to classical systemic lupus erythematosus.