We report the first case of profound "pseudohypocalcemia" after the administration of the gadolinium-based contrast media dimeglumine gadopentate (Magnevist; Schering Diagnostics AG, Berlin, Germany) to a long-standing hemodialysis patient. Initial calcium measurements were made by using our routine laboratory colorimetric method based on o-Cresolphthalein (Roche Modular-P; Roche Diagnostics Ltd, Lewes, Sussex, UK). Because the patient was well, with no signs of hypocalcemia, retrospective testing using both another o-Cresolphthalein colorimetric method (Roche Integra 800) and measurement of ionized calcium with an ion-selective electrode were performed, and confirmed laboratory pseudohypocalcemia.