In a prospective study carried out in Lyon, France, the association between the excretion of cytomegalovirus (CMV) and the increasing frequency and severity of viral respiratory infections in children attending day-care centers was evaluated. Urine samples were collected in November 1992 (S1) and 4 months later in February 1993 (S4). A total of 246 children aged 6-12 months attending 29 day-care centers from 1 November to 28 February were screened for the excretion of CMV in urine. The diagnosis of viral acute respiratory infection was performed in the case of outbreaks only. Forty-eight (19.5%) children were both S1 and S4 positive for CMV, 30 (12.4%) became CMV positive (S1-/S4+), 4 (1.6%) became negative (S1+/S4-) and 164 (66.7%) remained negative. The percentage of children becoming CMV positive was significantly (P<0.001) higher in day-care centers where more than 40 children were enrolled. Nine outbreaks due to respiratory syncytial virus, rhinovirus and enterovirus were recorded in 8 of 29 (27.6%) day-care centers. Viral acute respiratory infections were significantly (P<0.05) more frequently recorded in day-care centers in which CMV and respiratory viruses cocirculated and were significantly (P<0.001) more frequently reported in CMV-infected children. These findings suggest that viral acute respiratory infections are significantly more likely to occur in CMV-infected children.