A double blind vs placebo study was carried out to study the effect of letosteine on the symptoms and clinical course of paediatric patients suffering from acute febrile bronchitis. Forty children were recruited for the research: 20 were treated with letosteine in a dose of 25 mg x 3 g/die and 20 with placebo; treatment lasted 10 days. The following parameters were assessed during the trial: body temperature, cough, thoracic objectivity, respiratory function indices. The results of the study show that in the letosteine treated group there is a statistically significant decrease in fever, a favourable evolution of thoracic objectivity and an improvement in certain respiratory function parameters (MEF 75, PEF). It is concluded that treatment with letosteine leads to a significant increase in the rate of regression of thoracic symptomatology and a faster, more substantial reduction in fever in children suffering from acute bronchitis. This is probably the result of drug action on mucus viscosity, restoring optimal mucociliary clearance, and through action fostering the penetration of antibacterial substances into the mucus.