OBJECTIVE Recurrent upper respiratory tract infections (URTIs) are common illnesses in young children. As the immunoactive bacterial extract OM-85 has been shown to prevent these infections in both adults and children, the aim of the present trial was to investigate further its efficacy and safety in infection-prone children. METHODS This is a randomized, double-blind, placebo-controlled, multicenter study with OM-85 in 232 patients aged 36 to 96 months with recurrent URTIs. Treatment was one capsule daily during month 1 and during 10 days in months 3 to 5. URTI was defined by the presence of at least two of the following: rhinitis, pharyngitis, cough, hoarseness, temperature > or = 38.5 degrees C, or URTI-related prescription of an antibiotic. RESULTS OM-85-treated patients had a lower rate of URTIs (p < 0.05). The cumulated difference in URTIs between the two groups reached - 0.40 URTIs per patient in 6 months, corresponding to a 16% reduction in the active-treatment group with respect to placebo. The largest difference was observed in the patients having had three or more URTIs during the study period; odds ratios for three or more URTIs were 0.51 (95% confidence interval, 0.29 to 0.91) and 0.65 (95% confidence interval, 0.37 to 1.11) after 5 months and 6 months, respectively. The difference between OM-85 and placebo was independent of age but was more important in patients reporting a larger number of URTIs in the previous year. Patients' global assessment showed improvement in comparison to the previous season in the majority of the cases (OM-85, 78.4% of cases; placebo, 75.5%); however, there were more cases reporting worsening with placebo (6.4% vs 0.9%; p = 0.05). CONCLUSIONS OM-85 treatment significantly reduced the rate of URTIs, particularly in children with a history of frequent URTIs. Safety and tolerance of test medication were good, comparable to placebo.