Thirty children (25 males, five females), mean age 9.33 ± 2.57 years, with acute infectious pulmonary diseases or acute flare-ups of chronic bronchitis, were randomly treated with either domiodol or placebo. The study was conducted between November 1986 and February 1987. Domiodol was administered orally for 14 days in doses of 0.5 mg/kg three times daily. Subjective symptoms (cough, sputum viscosity, difficulty in raising sputum and sputum characteristics) were assessed semi-quantitatively. They were all reduced significantly in the group treated with domiodol compared to those given placebo. Spirometric measures of vital capacity and peak expiratory flow rate were also improved in patients given domiodol compared with placebo. Conversely, no differences were noted in forced expiratory volume in 1 s, Tiffenau index, or mid-expiratory flow. There were no clinical side-effects and no changes in any other parameters measured; in particular, levels of thyroid hormone and thyroid stimulating hormone were unchanged. Thus, domiodol proved effective in reducing the symptoms of infection of the lower respiratory tract and in improving the consistency of mucus in children.