• We studied 182 sick, febrile (temperature >38°C) infants less than 3 months of age, who presented at our Tripler Army Medical Center, Honolulu, during a one-year period, to determine the relative causes of fever in this age group. Blood, cerebrospinal fluid, urine, nasopharyngeal secretions, and stool specimens were cultured for bacterial and viral pathogens. Paired acute and convalescent sera were collected to serologically confirm infection in infants from whom viral isolations were obtained only from the nasopharynx or stool. A viral pathogen was isolated in 75 infants (41%) and a bacterial pathogen was isolated In 27 infants (15%). Nonpolio enteroviruses were the most common pathogens demonstrated. They were isolated from 64 infants (35%), and 40 (62%) of these infants had aseptic meningitis, the most frequently made diagnosis. Urinary tract infection was the most common bacterial infection observed. It occurred in 20 Infants (11%) and was most often seen without associated pyuria in uncircumcised male infants. Salmonellosis, the second most common bacterial infection, was observed in six infants (3%), and two of these did not have diarrhea or other gastrointestinal tract symptoms. No infant had septicemia and only one infant had bacterial (group B streptococcal) meningitis.