Foscarnet is a main treatment for disseminated cytomegalovirus infection in immunocompromised patients. One of its documented side effects is hypocalcemia. Hypercalcemia, in contrast, was described anecdotally before, almost exclusively in adults with human immunodeficiency virus infection or posttransplantation. We describe a case of severe hypercalcemia during foscarnet treatment in an infant with IL-7 Rα deficient severe combined immunodeficiency, resolved after treatment cessation. We speculate that this unusual side effect is caused by foscarnet binding to the inorganic matrix of bone.