The medical records of the cystic fibrosis patients at the CF clinic in Hannover are followed up by a computer-assisted on-line documentation. An overview of clinical and bacteriological data is given. During the last few years Pseudomonas aeruginosa has taken the place of Staphylococcus aureus as the major bacterial pathogen in the respiratory tract of CF patients. The chronic colonization with P. aeruginosa for more than three years correlated with excessive under-weight and failure to grow. As an example of a longitudinal study the beneficial effect of long-term aerosol administration of tobramycin on the clinical course of chronically colonized CF patients is demonstrated.