The efficacy and safety of a new synthetic immunostimulant pidotimod ((R)-3-[(S)-(5-oxo-2-pyrrolidinyl) carbonyl]-thiazolidine-4-carboxylic acid, PGT/1A, CAS 121808-62-6) in recurrent infections of the primary airways were assessed in a group of 416 children with a history of recurrent respiratory infections (RRI). This was a double-blind randomized trial of pidotimod vs. placebo, consisting of a treatment period of 60 days and a follow-up period of 3 months. A reduction in the duration and frequency of infectious episodes in the group of children treated with pidotimod (one 400 mg oral bottle daily) was observed which was statistically different from the placebo group. The protective effect produced by pidotimod was also confirmed by a series of recordings made over the five-month observation period, which showed a significant reduction in the number of days of fever, the severity of the signs and symptoms of acute episodes, use of antibiotics and antipyretic drugs and absence from school or nursery school. Safety was excellent.