Cystic fibrosis (CF) is one of the most common life-shortening autosomal recessively inherited diseases among the Caucasians, with an incidence of about 1 in 2500. In Estonia the incidence rate is 1 CF patient in 7700 live births. CF is a generalized exocrinopathy characterized by chronic progressive obstructive pulmonary disease, intestinal malabsorption secondary to pancreatic insufficiency and increased levels of sweat electrolytes. It is caused by mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene within region q31 on chromosome 7. Defects of the protein product of this gene result in failure of ion transport across the respiratory and exocrine gland cell epithelium, with water being osmotically retained in the cells. The mucos becomes 30–60 times more viscous than normal and stasis of secretions with infective sinusitis ensures. Persistent infection may be the cause of the characteristic chronic hyperplastic mucosal changes and polyps formation. Nasal polyps are rare under the age of 5, but are found in 26–56% of CF patients above the age of 5. We present a case of a 5-year-old boy with nasal polyps and cystic fibrosis. The patient presented to the otolaryngologist with a history of recurrent sinus infection and enlarged adenoids. The main presenting symptoms were chronic rhinosinusitis and obstructive polypoid masses within both nasal cavities detected by direct visualisation, which had been confirmed by computed tomography. He had normal height and weight growth without manifestations of pancreatic insufficiency or chronic endobronchial infections. Spirometry was normal and skin prick tests to common inhalant allergens were negative. Sweat chloride concentrations ranged between 71.6 and 75.7 mmol/l, identifying cystic fibrosis in this patient. Genetic analysis showed that the patient had heterozygous mutation 394delTT in the CFTR gene. CF should be considered in any child with nasal polyps and sweat electrolyte analysis must be done.