Recurrent respiratory tract infections are common in patients of all ages and are associated with high morbidity and high rates of medical consultations. Immunotherapy has long been used to boost the immune system and to confer protection against microbial infections. Ribosomal immunotherapy has been shown to stimulate both specific and nonspecific immune responses, particularly the mucosal immune defense system. The aim of this study was to evaluate the efficacy and tolerability of an oral ribosomal immunostimulant (Immucytal® tablets) in the prevention and treatment of respiratory tract infections. This was a multicenter (n = 3), randomized, placebo-controlled, double-blind, parallel-group study A total of 180 adult patients with recurrent upper respiratory tract infections (URTIs) or bronchitis were recruited from university clinics or hospitals. Immucytal® or placebo was administered according to the following schedule: 1 tablet/day for 4 days/week for 3 consecutive weeks (first month), followed by 1 tablet/day for 4 days every month for the following 5 months. The main efficacy parameter was a semiquantitative evaluation by a clinical score of clinical efficacy. The following items were considered: incidence of upper and lower respiratory tract infections, fever, number of medical consultations and adjunctive treatments required. At the end of the study the clinical score showed a significant reduction in patients treated with Immucytal® compared with placebo for all the most important items considered, starting at 2 months after treatment initiation. Concomitantly there was a significant change in immunological parameters (increased plasma levels of IgG, IgE and IgA titers, higher counts of CD4+ and CD8+ lymphocyte subpopulations) and an improvement in respiratory function parameters. A significantly higher proportion of physicians and patients expressed a preference for Immucytal® vs. placebo (p <0. 001). Immucytal® was very well tolerated and the incidence of adverse effects was similar in the two treatment groups. These results show that oral Immucytal® is efficacious and safe in patients with recurrent URTIs or bronchitis.