In a randomized trial in 102 preterm newborns with respiratory distress syndrome (RDS) it has been shown that early Ambroxol treatment (30 mg kg(-1) over the first 5 days) significantly reduces the incidence of RDS-associated complications [bronchopulmonary dysplasia (BPD), intraventricular haemorrhage, post-natal acquired pneumonia]. The aim of the present analysis was to investigate the effect of Ambroxol treatment on lung function in newborns who developed BPD. Respiratory function testing (RFT) was performed immediately after extubation and at day 28. Tidal volume (VT) and respiratory frequency (f) were measured during tidal breathing using the deadspace free flow-through technique. The lung mechanic parameter VT/maxPes was determined by measuring the maximal oesophageal pressure changes, maxPes, with a catheter tip pressure transducer. In the placebo group 36/50 infants were extubated within the first 28 days of life and 13/36 (36%) developed BPD. In the Ambroxol group 44/52 were extubated and 9/44 (20%) developed BPD. After extubation, RFT showed (i) no statistically significant difference in the ventilatory parameters of either treatment group, (ii) improved (P<0.05) lung mechanics (VT/maxPes) in Ambroxol group compared to controls (94+/-27 ml kPa(-1) vs. 8.1+/-2.6 ml kPa(-1)) and (iii) no statistically significant difference in lung function between infants with and without BPD. At day 28 we found (i) no effect of early Ambroxol treatment on lung functions, (ii) significantly (P < 0.05) higher f (58.5+/-11.7 min(-1) vs. 49.7+/-10.1 min(-1)) and significantly (P<0.01) lower V(T) (9.6+/-1.9 ml vs. 12.3+/-2.7 ml) and V(T)/maxPes (8.9+/-2.6 ml kPa(-1)] vs. 12.0+/-2.9 ml kPa(-1)) in infants with BPD compared to infants without and (iii) these differences are not influenced by early Ambroxol treatment. If the process of BPD development is induced, early Ambroxol treatment has no influence on impaired lung function at day 28.