Research goals: To analyze and determine diagnostic aspects of cystic fibrosis (CF), including clinical manifestations of subjects at the Pediatric Clinic, Clinical Hospital Center Rijeka. In addition to the demographic data of the patients, age and gender, determine the average age of diagnosis of CF. To show the most common respiratory tract infections depending on the age of the patient and to determine the annual fall in the forced expiratory volume in the first second (FEV1) based on the spirometry value. Research subjects: This study included 14 CF patients who have been diagnosed and treated in the period from July 2010 to April 2017 at the Pediatric Clinic of KBC Rijeka. Results: The average age of diagnosis of CF was 10, 5 months. In the largest number of diseased, diagnostic treatments started because of a weight loss, and only in one patient because of the respiratory symptoms. It was found that the most common mutation is ΔF508. Six patients were diagnosed with sensitization to inhaled allergens, and the two of them also showed a sensitization to the mildew Aspergillus fumigatus. ABPA was present in one patient. The most common isolate from the upper and lower respiratory system, up to approximately 16 years, was Staphylococcus aureus, and then the major airway colonizer became Pseudomonas aeruginosa. The average annual drop in pulmonary function was 1%. Conclusion: Diagnostic aspects of CF are based on a diagnostic consensus which may suspect the disease and then lead to the diagnosis. Early recognition of the disease by the pediatric specialist, and persistent and consistent implementation of therapeutic measures by the patient, results in a longer and better life.