30 aged patients, with chronic urinary tract infections, were treated with a thymus extract (thymostimulin, TP-1 Serono), in combination with antibiotic therapy, according to the antibiogram data, or with antibiotic therapy alone. When thymus extracts were added to the treatment, an improvement of the clinical course of the disease was observed and the severity and duration of the subjective symptoms such as strangury, dysuria, and pollakiuria, were reduced. The two groups of patients presented no modifications of the febrile patterns, nor persistence in or recovery from infection after 30 days of treatment, but especially after 90 days, the frequency of reinfections or the persistence of infections were markedly reduced. During treatment with thymostimulin, there was an evident increase in the cellular immunity tests (T-lymphocytes, rosettes, PHA) as compared with the immunodepression found before treatment.