The activity of pidotimod ((R)-3-[(S)-(5-oxo-2-pyrrolidinyl) carbonyl]-thiazolidine-4-carboxylic acid, PGT/1A, CAS 121808-62-6) was evaluated in a double-blind, placebo-controlled, randomized, multicentre trial, on 120 pediatric patients affected by recurrent respiratory infections. The clinical course of acute infections was favourable both in placebo and in treatment group, but recovery was quicker with pidotimod than with placebo. Antibiotic therapy and time of hospitalization were shorter in the patients taking pidotimod, and main symptomatic parameters (pharyngalgia, dysphagia, mucous membrane inflammation, adenopathy, anorexia) receded quickly. In patients receiving the drug as well as in placebo group changes in laboratory parameters, indicating recovery from the acute infectious events, were observed. A significant trend to normalization of the immune response, evidenced by chemotaxis and leukocyte phagocytosis index, was found only in patients treated with pidotimod. A significant decrease in the risk of relapses was observed in patients treated with pidotimod (35%), as well as a reduction of hospitalization (86%) and a decreased antibiotic therapy (47%). If a relapse occurred, the response of treated patients was quicker (fever, antibiotic therapy, hospitalization). These findings allow to correlate the individual immune response activation to the resistance to recurrent infections and also to a better response to therapy in case of clinically relevant disease. No side effects were observed. Only in 12 patients (5 pidotimod, 7 placebo) mild reactions were observed, but they were attributed to concomitant antibiotic treatment or other factors. No alterations in main laboratory parameters were seen.(ABSTRACT TRUNCATED AT 250 WORDS)