Neonatal lupus erythematosus (NLE) is an uncommon disease that is manifested by cutaneous lesions, cardiac conduction defects, or both, that appear in utero or shortly after birth. In approximately 95% of patients, anti-Ro antibody (Ro[SS-A]) has been identified and has become the serologic marker for NLE. Since 1987 there have been four reported cases of Ro- and anti-La antibody (La[SS-B])-negative, U1RNP antibody-positive, NLE. Our affected twin, as well as all other infants with U1RNP-positive NLE, had cutaneous lesions similar to those in Ro-positive NLE, although they lacked systemic abnormalities, including cardiac conduction defects. HLA typing of mothers with infants with U1RNP-positive NLE revealed the presence of HLA-DR4, DQw1, or DQw3 phenotypes. Our typing confirms these findings. As with Ro-positive NLE, no distinct HLA associations were demonstrated in the infants. Unlike Ro-positive mothers, all mothers with a U1RNP-positive infant with NLE had connective tissue disease at the time of the diagnosis and had a different spectrum of disease. We describe the clinical, serologic, and immunogenetic findings in the first reported case of U1RNP-positive NLE in dizygotic twins in whom the NLE disease expression was discordant.