The diagnosis of cystic fibrosis is based on the occurrence of clinical symptoms and evidence of two mutations in cystic fibrosis transmembrane conductance regulator. However, in cases of atypical cystic fibrosis, not all symptoms may be manifested and not all diagnostic tests may be positive. The World Health Organization has recognised the need for a category that covers patients with atypical (often single organ disease), who may or may not supply evidence for cystic fibrosis transmembrane conductance regulator dysfunction (sweat test) or two cystic fibrosis transmembrane conductance regulator mutations. The aim of this case report is to highlight the difficulties in cystic fibrosis diagnosis in some groups of patients who do not fit in the “gold standard” of the diagnostic scheme. Individuals with atypical cystic fibrosis might only have dysfunction in one organ system and of a much milder degree than those with typical cystic fibrosis. This may be the main reason for a diagnosis and treatment delay.