The activity of pidotimod ((R)-3-[(S)-(5-oxo-2-pyrrolidinyl) carbonyl]-thiazolidine-4-carboxylic acid, PGT/1A, CAS 121808-62-6) was studied vs. placebo in a double-blind, randomized, multicentre trial, involving 60 pediatric patients with recurrent urinary tract infections. Recovery from acute events was quicker with pidotimod than with placebo (9.6 vs. 12.3 days). In treated patients antibiotic therapy was shorter (6.9 vs. 8.3 days) and main symptomatic parameters (body temperature, vesical tenesmus, stranguria, pollakiuria, total number of symptoms, total symptomatic intensity, rate of asymptomatic patients, haematuria, leukocyturia, positive urinary culture) receded quickly. In patients receiving the drug as well as in patients treated with placebo changes in laboratory parameters were observed, indicating recovery from the acute infectious disease. A significant trend to normalization of the immune response, expressed by chemotaxis and index of leukocyte phagocytosis, was found only in patients treated with pidotimod. After the acute episode a significant decrease of risk of relapses (69%) was observed in these patients. If a relapse occurs, the response of treated patients is quicker (duration of fever, total time of relapses) than for control patients. These findings allow to correlate the individual immune response activation to the resistance to recurrent infections and also to a better response to therapy if the disease occurs and becomes clinically relevant. No side effects were observed. Mild reactions (4 nausea/vomiting, 1 erythema) occurred only in 5 patients (2 pidotimod, 3 placebo) but were attributed to concomitant antibiotic therapy. No alterations of main laboratory parameters were found. These findings confirm the tolerability of the drug also in long-term treatment.(ABSTRACT TRUNCATED AT 250 WORDS)