A prospective study was conducted to determine the frequency and distribution of bacterial and viral pathogens in infants hospitalized with suspected sepsis and to evaluate the potential of virus detection for improving patient management. A causative organism was detected in 157 (67%) of 233 previously healthy infants less than 3 months of age, who had been hospitalized for suspected sepsis: 19 (8%) had bacterial infections, 135 (58%) had viral infections, and 3 (1%) had mixed viral-bacterial infections. Viral infections occurred in a seasonal pattern: enteroviruses were responsible for most of the hospitalizations during summer and fall (65/110; 63%) and respiratory syncytial and influenza A viruses were responsible for most of the infections during winter (44/81; 55%). In contrast, bacterial infections were not seasonally distributed. Virus was detected in 33% of the 138 infected infants within 24 hours, and in 64% within 3 days. We conclude that viral infections are prevalent among infants hospitalized for suspected sepsis, and most can be detected early enough to influence patient management.