In the Houston Family Study, young children were cultured for virus weekly or biweekly and during acute respiratory illnesses. The interval between the onset of illness and positive culture was examined for 179 infections during 1975-1979. In week 1 after onset, 73%, 73%, and 66% of cultures were positive for influenza A virus, respiratory syncytial virus (RSV), and parainfluenza virus type 3, respectively. Pooled data from influenza B virus infections in 1977 and 1980 showed that 73% of cultures were positive in week 1. Influenza A virus in week 2 or RSV in weeks 2 and 3 was isolated from very few children. However, 37% of cultures were positive for influenza B virus during week 2, and 17% of cultures were still positive for parainfluenza virus type 3 during week 3. Shedding of parainfluenza virus type 3 on days 29-38 was also observed. Parainfluenza virus type 3, RSV, and influenza A virus were isolated up to six days before the onset of illness.