A longitudinal study of symptomatology and interferon production in 35 patients (13 children, 20 college students and 2 medical staff members) with influenza B viral infections is presented. The mean duration of illness was 11.3 days. Cough persisted for 9.8 days and nasal symptoms continued for 8.5 days. The mean duration of fever was 4.7 days. Diarrhea occurred in 8 patients; it had its onset on about the fourth day and persisted for 4 days. Interferon levels correlated directly with fever and generally with systemic symptoms but not with other aspects of disease severity, duration or serum hemagglutination-inhibiting antibody responses.