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 3.1–22.6y. 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 (15mg/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.