Cystic fibrosis (CF) is a recessive genetic disease caused by defects of the cystic fibrosis trans-membrane regulator (CFTR) gene with a median survival of less than 35 years. This work reports on the oldest living German siblings with CF. Besides clinical history, CF genotype and nasal potential difference (NPD) measurement results, the remarkably high exercise activity of the siblings is discussed as a disease-modifying factor. Both male patients have an overall mild pulmonary manifestation. They have suffered from abdominal symptoms since their early childhood, including recurrent pancreatitis and diffuse symptoms leading to partial gastric resection. They were diagnosed as having CF with positive sweat tests at the advanced ages of 45 and 43 years, respectively. Later on genotyping revealed compound heterozygosity for F508del and 2789+5G-->A. Using NPD we demonstrated a CF-typical inhibition of the NPD by the Na channel blocker amiloride, although in both siblings the remaining CFTR function and alternate chloride channel function were detected during superfusion of the nasal epithelium with isoproterenol and ATP. Long-term survival with CF is basically influenced by the CFTR genotype. The patients' genotype was discussed as a mild one with remaining CFTR function. We demonstrated this residual CFTR function in both siblings using NPD. Additionally the siblings' continuous healthy lifestyle and their engagement in a remarkably high level of exercise activities from early childhood to the present possibly have an important effect on the long-term outcome of CF as disease-modifying factors. In this regard this report can encourage CF patients to maintain a high level of physical activity in their daily lives.