Mutations in the HNF1B gene represent an important cause of kidney disease and monogenic diabetes of the MODY type. These mutations are characterised by marked phenotypic variability – ranging from isolated cystic changes in the kidneys to complex syndromes involving metabolic disturbances, diabetes, and developmental anomalies of other organs. In this report, we describe the case of an 8.5-year-old boy in whom the first and only clinical manifestation consisted of renal cystic changes. Molecular testing revealed a pathogenic mutation in the HNF1B gene, confirming the genetic basis of the disease. This case exemplifies the diagnostic challenges posed by the broad phenotypic spectrum of HNF1B-related disorders and highlights the fact that renal abnormalities may precede the onset of other clinical features. The patient was placed under nephrological, diabetological, and genetic follow-up, with further management focused on monitoring renal function, electrolyte balance, and glycaemic control. This case underscores the necessity of including HNF1B mutations in the differential diagnosis of renal cystic disease, emphasises the pivotal role of genetic testing in identifying the underlying aetiology, and demonstrates the importance of early recognition for ensuring comprehensive and personalised care.