Background/Aims: Several recent clinical studies revealed an accumulation of ceramide in bronchial epithelial cells of patients with cystic fibrosis (CF). Degradation of ceramide concentrations in lungs of CF patients employing the functional acid sphingomyelinase inhibitor amitriptyline revealed a benefit in lung function, weight and exacerbation rates. Methods: To test for a beneficial effect of amitriptyline in vivo, we performed two phase II randomised, double-blind, placebo-controlled studies. CF patients were treated with 25 mg amitriptyline twice daily, i.e. a total dose of 50 mg/d. After those two studies part of the patients used amitriptyline in an off-lable-use for routine treatment. These patients were observed after one, two and three years after continuous use of amitriptyline and were matched with those patients who were not treated. These patients were used as a control group. Results: After one year of treatment, forced expiratory volume in 1 sec predicted (FEV<sub>1</sub>) increased significantly by 7.6±7.0%, p=<0.001, and weight increased by 2.1±2.3kg, p=<0.001 in the amitriptyline population (n=20), whereas FEV<sub>1</sub> decreased significantly in the control group by 1.8±3.3%, p=0.010, and weight increased by 1.1±2.7kg, p=0.010 (n=14). After two years of treatment, FEV<sub>1</sub> increased significantly by 5.6±10.3%, p=0.009, and weight increased by 3.6±2.9kg, p=<0.001 in the amitriptyline population (n=12). In contrast, FEV<sub>1</sub> decreased in the control group by 2.1±3.7%, p=0.051 and weight increased by only 0.4±2.9kg, p=0.31 (n=10). After three years of treatment, FEV<sub>1</sub> increased significantly by 7.7±8%, p=0.050, and weight increased by 7.3±3.8kg, p=0.016, in the amitriptyline population (n=5), whereas FEV<sub>1</sub> decreased in the control group by 1.0±1.3%, p=0.075 and weight increased by 0.4±1.5kg, p=0.29 (n=5). Conclusion: Amitriptyline significantly increases FEV<sub>1</sub>, reduces ceramide in lung cells and increases weight of CF patients.