In order to test the ability of Ambroxol to improve the clinical course of respiratory distress syndrome and to reduce the incidence of complications a multicentre, randomized, placebo-controlled double-blind trial was conducted. Entry was limited to infants with a birth weight below 1500 g. A total of 179 neonates were enrolled, but 31 were later excluded because they had other diseases. Of the remaining 148 babies, 74 received Ambroxol (birth weight 1190 +/- 216 g; gestational age 29.1 +/- 1.9 weeks) and 74 placebo (birth weight 1168 +/- 216 g; gestational age 28.9 +/- 1.9 weeks). In the Ambroxol group 23 (31%) and in the placebo group 27 (37%) infants died during the first 5 months of life. In 28 day-survivors Ambroxol was able to significantly improve the PaO2/FiO2 ratio, mean airway pressure, phospholipid profile of tracheal effluent and pulmonary mechanics of spontaneously breathing infants. In addition, the incidences of bronchopulmonary dysplasia (29% vs 54%), intraventricular haemorrhage (25% vs 44%) and postnatally acquired pneumonia (15% vs 36%) were significantly reduced in the Ambroxol group as compared to the control group. No adverse events attributed to the Ambroxol treatment were reported.