Hypercalcemia in HIV patients has been previously reported, but 1,25-(OH)2vitamin D-mediated hypercalcemia, due to increased activity of extrarenal 1-alpha hydroxylase, is rarely described with HIV-related infections or malignancies. We describe a case of 1,25-(OH)2vitamin D-mediated hypercalcemia in a patient presenting with progressive cognitive decline and weakness. Initial evaluation revealed a new diagnosis of HIV, for which he was started on antiretroviral therapy (ART). He was also noted to have mild asymptomatic hypocalcemia, likely from his acute illness and malnutrition, which was not further investigated at the time. While the patient's mental status initially improved with ART, he became progressively delirious and was found to be hypercalcemic approximately 4 weeks after the initiation of ART. Possible etiologies for hypercalcemia were vigorously evaluated, including granulomatous disease, infection, and malignancy, in the setting of suspected immune reconstitution inflammatory syndrome (IRIS), due to recent initiation of ART. Infectious workup was unrevealing, but computed tomography (CT) of the chest, abdomen, and pelvis revealed new extensive diffuse lymphadenopathy and hepatomegaly, not present on admission studies. Cytology and flow cytometry of a liver biopsy specimen revealed CD10 positive high-grade B-cell lymphoma. Chemotherapy was not pursued due to poor performance status. Over the next week, spontaneous tumor lysis developed, and the patient expired. Postmortem, his 1,25-(OH)2vitamin D level returned as markedly elevated. Immunohistochemical staining of his liver biopsy tissue showed strong expression of CYP27B1.