Viral investigations were performed during 4 winter seasons (88/89, 89/90, 92/93, 93/94) in children attending day-care centers (DCCs) in the Rhone Département in eastern France. Over the total observation period of 4 winter seasons, 780 children were screened with a nasal swab for the presence of viruses. Of those, 230 (29.5%) had a positive viral culture. The viruses identified were respiratory syncytial virus (RSV), influenza A and B virus, parain-fluenza virus, coronavirus, rhinovirus, adenovirus and enterovirus. During that time, 83 epidemic events in 47 DCC were recorded. A particular virus was judged to be causally related to an epidemic if the identical virus was isolated in ≥ 3 children during the same outbreak of respiratory diseases. Thus, in 51 cases (61.4%) of all epidemics, the following viruses were responsible for an epidemic: RSV (n = 23), coronavirus (n = 10) (only during the season of 1993-1994), influenza A virus (n = 6), rhinovirus (n = 4), enterovirus (n = 4), adenovirus (n = 3) and parainfluenza virus (n = 1). Except for the somewhat surprising accumulation of coronavirus epidemics during the winter of 1993-1994, there were only minor seasonal variations from one year to another. As expected, RSV accounted for about one third of all respiratory tract infections in children attending DCCs and was therefore the most important single causative agent. These results are compared with data from children who did not attend a DCC and were cared for in a private practice. During the winter of 1989-1990, the viral epidemiological survey was performed at the same time and in parallel to a double-blind, placebo-controlled clinical study investigating the efficacy of OM-85 BV, an immunoactive bacterial extract. This study, enrolling 423 children attending DCCs demonstrated a protective effect of OM-85 BV in significantly reducing the risk of recurrent infections of the upper respiratory tract during the treatment period with the compound. 34% of all participating children (75 in the verum group, 70 in the placebo group) were enrolled in an additional virological study. In these patients, RSV was isolated 10 times in the placebo group, but only 5 times in the treated group (p < 0.05) and influenza A virus was present in 4 children in the placebo group, but only in 1 infant in the verum group giving a total of 14 positive virologie results in the placebo group versus 6 in the verum group (p < 0.05). Despite the small numbers of children investigated for their virologie status during respiratory infectious outbreaks, there was a statistically significant difference in the prevalence of virus carriers in favor of the children treated with OM-85 BV. These results corroborate the clinical findings.