Enteroviruses generally cause mild disease; however, neonates are at higher risk for severe illness because of the immaturity of their immune systems. Neonatal systemic enterovirus disease, characterized by multiorgan involvement, is among the most serious, potentially fatal conditions associated with enterovirus infection. Typical clinical presentations include encephalomyocarditis (characteristic of group B coxsackieviruses) and hemorrhage-hepatitis syndrome (typical of echovirus 11). To describe the severity of neonatal illness associated with coxsackievirus B1 (CVB1) infection, CDC analyzed case reports and preliminary data from the National Enterovirus Surveillance System (NESS) for 2007. This report describes the results of that analysis, which indicated that, in 2007, CVB1 for the first time was the predominant enterovirus in the United States, accounting for 113 (25%) of 444 enterovirus infections with known serotypes. In addition, phylogenetic analysis of the 2007 CVB1 strains suggested that the cases resulted from widespread circulation of a single genetic lineage. Health-care providers and public health departments should be vigilant to the possibility of neonatal disease caused by CVB1. Testing for enteroviruses in clinically compatible cases and reporting of identified enteroviruses to NESS should be encouraged.