Enteroviruses were isolated from feces and/or cerebrospinal fluid of 29 of 43 Toronto children who contracted aseptic meningitis, pleurodynia, abdominal pain or febrile upsets between June and October, 1965. Coxsackie A9 virus was the dominant agent in aseptic meningitis and Coxsackie B1 virus in pleurodynia and other syndromes. Sero-logical evidence of recent Coxsackie B1 and Echo 6 infection was obtained in two additional patients with aseptic meningitis who did not yield virus, and elevated Coxsackie B1 antibody titres were found in one patient with pericarditis. A newborn infant died with myocarditis due to Coxsackie B1 virus following infection of the mother during the immediate antenatal period. Paired sera collected only two to four days apart from patients with enteroviral syndromes or mumps meningoencephalitis frequently showed four-fold or greater increases of antibody levels.