50 young patients suffering from recurrent respiratory infections (RRI) were treated with pidotimod ((R)-3-[(S)-(5-oxo-2- pyrrolidinyl) carbonyl]-thiazolidine-4-carboxylic acid, PGT/1A, CAS 121808-62-6) (one 400 mg ampoule twice a day) or placebo, according to a double-blind experimental design. The treatment period was 20 days and there was then a 60-day follow-up period. Evaluation was both clinical (number and severity of the respiratory infectious episodes) and immunological, investigating the OKT 4 and OKT 8 lymphocyte sub-populations and OKT 4/OKT 8 ratio. The group of children treated with pidotimod showed a decrease in the number of infections. Patients free from RRI episodes, after 20 days of therapy, were 68% of the group treated with pidotimod compared with 8% of the placebo group. In addition, the mean duration of the episodes was lower in treated patients than in patients of the control group. Such differences were statistically significant. It was also observed that administration of the drug potentiated the immune response such that the clinical picture remained improved for a further 60 days after treatment cessation. Furthermore, only in the pidotimod group there were improving changes of OKT 4 and OKT 8 percentages which affected the OKT 4/OKT 8 ratio.