Antibodies to SS-A/Ro have been proposed to be a serologic marker for the neonatal lupus syndrome, which is characterized by congenital heart block or cutaneous lupus or both. The antibodies occur in the mother and are transiently found in the child's serum. We examined an unselected series of 12 children with idiopathic CHB, isolated in 10 children and with cutaneous lupus lesions in two. Six of these children and their mothers were studied during the child's neonatal period, and six were studied retrospectively. All six neonates had SS-A/Ro autoantibodies. Nine of 12 mothers had SS-A/Ro autoantibodies. Of the seropositive mothers, one had systemic lupus erythematosus, two had sicca syndrome, one had photosensitivity, one had arthralgias, and four were asymptomatic. We propose that congenital heart block may be related to transplacental passage of maternal SS-A/Ro antibodies and that neonatal lupus may be the most common cause.