Cystic fibrosis-related diabetes is the most common comorbidity of cystic fibrosis patients. It may occur at any age, even in infancy, but the prevalence increases as patients get older. We present a case of a female patient (15 years old) and a male patient (17 years old) hospitalized at the Cystic Fibrosis Center. The patients had lung symptoms as chronic daily cough with mucopurulent, yellow dark viscous sputum in large quantity, intensefied in the morning, with recurrent streaks of blood, dyspnea during moderate physical effort and permanent wheezing. The main complaints were polyuria, polydipsia and after the laboratory tests were performed, they were diagnosed with cystic fibrosis-related diabetes. Cystic fibrosis-related diabetes can be frequently overlooked in clinical practice due to cystic fibrosis symptoms.