Diabetes insipidus (DI) of central origin is caused by decreased or absent secretion of antidiuretic hormone. Neurogenic DI results from: brain tumors, neurosurgical interventions, congenital organic lesions, genetic defects, trauma, inflammatory or infiltrative processes. We describe a 19-year-old boy with a central DI caused by a sellar-suprasellar tumor. Based on characteristic history (4 years of polydipsia and polyuria), biochemical tests and imaging studies, DI was diagnosed in this boy at the age of 15 years. On his first MRI examination nodular thickening within the pituitary stalk was observed and treatment with desmopressin was started. After 4 months, MRI revealed a large tumor within a sellar-suprasellar region. Based on the characteristic localization of the tumor and increased serum concentration of biomarkers [human choriongonadotropin β (β-hCG) and alpha fetoprotein (AFP)] germ cell secreting tumor was diagnosed. The boy underwent chemo- and radiotherapy with a good outcome. Currently, at the age of 19 years, he is in remission. This patient is an example of diagnostic difficulties in case of DI, which can be caused by developing germ cell tumor. In each case of DI it is strongly advised to perform imaging studies (MRI) and repeat them when any abnormalities are found.