A 16-year-old boy was admitted to our hospital because of coughing, sputum, and exertional dyspnea. Seven months after birth cystic fibrosis had been diagnosed. The chest roentgenogram on admission showed diffuse reticulonodular shadows and overinflation. Pulmonary function tests revealed obstructive and restrictive impairment. Erythromycin and Lomefloxacin were administered by mouth, and aminoglycosides were administered by inhalation. His symptoms were alleviated, and he is now an outpatient. In Japan, cystic fibrosis is rare, and this patient is extremely rare because he has grown up to be a 16-year-old. In this case, low-dose and long-term erythromycin administration was very effective.