A randomized, double blind, placebo-controlled clinical trial was performed in 423 children attending day-care centers to assess whether stimulating nonspecific immunity would reduce the incidence of recurrent infections. The drug used for the trial (Imocur) is an extract obtained from eight different species of bacteria. At the end of the total follow-up period (3 months with treatment and 4.5 months without), the risk for > or = 4 episodes of upper respiratory infections was not significantly lower in the treated group than in the placebo group (26.7% vs. 33.8%, relative risk, 0.79; 95% confidence interval, 0.59 to 1.06). In an exploratory analysis limited to the 3-month treatment period, however, we observed a 48% reduction in the risk of presenting > or = 3 episodes of upper respiratory infections: 9.5% vs. 18.3%, respectively, in the treatment group and the placebo group (relative risk, 0.52; 95% confidence interval, 0.31 to 0.86). Similar results were found for the risk of > or = 1 episode of gastroenteritis. We also observed a strong correlation between the drug efficacy and age; this observation is coherent with the underlying pathophysiologic model in which the immune system matures with age.