Acute lower respiratory illnesses were prospectively investigated in a cohort of 1246 healthy infants who were enrolled at birth in the Tucson Children's Respiratory Study and followed through the first 3 years of life. Respiratory syncytial virus (RSV) infection was documented by culture, antigen detection or both in 276 episodes. In 21 (7.6%) of these 276, other viruses were simultaneously detected. Further serologic studies of 50 episodes in which RSV had been found increased the apparent viral codetection rate to 24%. When culture results for Chlamydia trachomatis and Mycoplasma pneumoniae were also considered, the rate of codetection was found to be 10.9% (30 of 276); this increased to 28% for the subgroup of episodes (14 of 50) that was further studied serologically. Illnesses associated with more than one agent were not significantly different from those involving RSV alone, with respect to month of onset, age at illness, illness type or duration of illness. We conclude that when RSV has been detected in previously healthy infants, routine searches for the concomitant presence of other viruses are usually not warranted.