The aim of the study was to draw attention to the danger of vitamin D overdose. The beneficial effects of vitamin D are described in both scientific literature and popular science journals. On the other hand, papers on the symptoms of excess intake of vitamin D are scarce. Since vitamin D preparations are easily accessible, they are widely used in both adults and children without any medical supervision. We present a case of a 2-year-old boy with life-threatening hypercalcaemia due to excess vitamin D intake. The boy required prolonged hospital stay, intensive therapy to reduce blood calcium, complex pharmacotherapy and hemofiltration. Abdominal ultrasonography performed after therapy completion showed enlargement of both kidneys and persistent hyperechogenicity of renal cortex. Expert recommendations specify the route of administration and the dose of vitamin D. In the era of the Internet, however, opinions suggesting doses that are inconsistent with recommendations may be found. Such management may have serious health consequences and even be life-threatening in some cases.