Previous studies have suggested a role for anti-Ro antibodies in the pathogenesis of neonatal lupus erythematosus (NLE). We reexamined the role of the immune response to the Ro/La particle in the development of NLE using newer and more definitive assay techniques. All 15 infants with congenital heart block and NLE had both anti-Ro and anti-La antibodies. Of 8 patients with cutaneous NLE alone, 2 had anti-Ro antibodies alone while the remaining 6 had both anti-Ro and anti-La antibodies. One woman gave birth to 2 children; one had both Ro and La antibodies and developed congenital heart block, the other had only anti-Ro and anti-U1RNP antibodies and was clinically normal. Our study demonstrates that the presence of anti-Ro and anti-La antibodies define the immune response that is associated with the development of congenital heart block of NLE.