The differential diagnosis of hypercalcaemia in patients with end-stage renal disease undergoing dialysis should be considered for causes related or unrelated to renal failure itself or therapies for renal failure. In particular, peritoneal dialysis may hinder awareness of a clinical problem due to its own peculiarities and effects on homeostasis of the body, thus creating misconceptions in interpreting laboratory data and diagnosing a disease. We describe here a case of systemic sarcoidosis which was delayed due to failure to recognise underestimated hypercalcaemia in a patient undergoing peritoneal dialysis. Clinicians need to remain aware of the change of minerals that may arise from peritoneal dialysis and should perform an extensive investigation for the cause of hypercalcaemia.