Idiopathic infantile hypercalcaemia (IIH) is a mineral metabolism disorder of unknown origin. It is characterized by high levels of serum calcium resulting in parathyroid hormone (PTH) suppression, muscle hypotonia, thirst, anorexia, failure to thrive, psychomotor retardation, constipation, nephrocalcinosis. Treatment consists of low calcium diet, glucocorticoids, furosemide. We present a case of 5-month old girl with IIH, where total calcaemia peaked to 4.25 mmol/l. The leading symptoms were failure to thrive, constipation, muscle hypotonia, dehydration. Rehydration, low calcium diet, and application of glucocorticoids and furosemide resulted in a drop in calcaemia to normal values and an overall clinical improvement within two weeks. Williams-Beuren syndrome (WBS), benign familial hypocalciuric hypercalcaemia (FHH), neonatal severe primary hyperparathyroidism (NSHPT), Jansen's metaphyseal dysplasia, primary hyperparathyroidism, vitamin D intoxication, granulomatous diseases, thyroid disease, malignancy were all ruled out. In conclusion, infants with failure to thrive should have their serum levels of minerals, especially, calcium, checked. In case of hypercalcaemia, treatment with corticosteroids and furosemide should be initiated, together with further diagnostic steps in order to elucidate its origin.