Klippel–Trénaunay syndrome is a rare congenital condition characterised by a triad of symptoms: capillary-lymphatic-venous malformations, varicose veins and venous malformations as well as soft tissue and skeletal hypertrophy of the affected limb. In this article, we present a case of a 5-year-old boy with extensive vascular malformations of the lower limbs and the buttock region. In this case, manifestation of all three symptoms was gradual. At the age of 4 years, the patient was admitted to the Department of Imaging Diagnostics for further diagnosis, where the triad characteristic for Klippel–Trénaunay syndrome and popliteal vein agenesis were diagnosed. Currently, a multidisciplinary team takes care of the boy in the Children’s Memorial Health Institute. Early and accurate diagnosis allows for rapid prevention of complications associated with Klippel–Trénaunay syndrome and enables patient-tailored treatment.