To discover the nature of enteroviral infections in early infancy (infants less than two months of age) in the United States, data were examined of 338 such infants with nonpolio enterovirus isolates reported to the Center for Disease Control in 1972-1975. Interpretation of such data is limited by reporting bias and difficulty in documenting disease causality. Enteroviral disease was severe in 74% of the infants. The most common clinical manifestations were meningitis, severe systemic disease, nonspecific febrile illness, and gastrointestinal symptoms. Fifty-one percent of all enteroviral disease was associated with echoviruses; 45% with Coxsackie B viruses; and only 4% with Coxsackie A viruses. Disease was more prevalent in the summer months, was recognized throughout the country, and occurred more frequently in males.