A "new pneumonia" is named legionnaires' disease after a gramnegative bacillus - Legionella pneumophila, unknown until 1977. Sofar it has been observed mostly in older male patients, rarely in children and never before in Germany. The legionnaires' disease in our patient, a five and half years old boy with cystic fibrosis caused a severe dyspnea and multilobular pulmonary densifications on the chest X-ray film. The diagnosis was verified by the rise of the titer in the indirect immune fluorescence test and the success of the therapy with erythromycin.