Most children with cystic fibrosis (CF) show persisting steatorrhoea even when treated with pancreatic enzyme. As a low duodenal pH could be responsible for this persisting fat loss, we evaluated the effects of a proton-pump inhibitor (lansoprazole) on both steatorrhoea and growth parameters in 15 CF patients, aged 3.1-22.6 y. Acid steatocrit, anthropometry and dual-energy X-ray absorptiometry were used to evaluate steatorrhoea and the nutritional status before, during and 3 months after stopping lansoprazole treatment (15 mg/d for 3 months). Mean +/- SD acid steatocrit values decreased from 37.1 +/- 8.8% to 28.5 +/- 10.6% (p = 0.02). Significant mean Z-score improvements were found for weight (+0.14; p = 0.02), height (+0.15; p = 0.03), subscapular (+0.61; p = 0.003), supra-iliac (+0.8; p = 0.002) and the sum of the four measured skinfolds (+0.61; p = 0.002). Z-scores deteriorated again after stopping lansoprazole. Fat mass and bone mineral content increased significantly on lansoprazole (p = 0.008 and p = 0.005, respectively). We conclude that lansoprazole as adjuvant therapy significantly improves both steatorrhoea and the nutritional status in CF children who maintain steatorrhoea while on pancreatic enzymes.