Abstract: The author reports a patient who was admitted due to 'pain in the lumbosacral region and bilateral lower limbs with limited activity for 1 year'. Based on clinical symptoms, laboratory tests, imaging examinations, bone marrow cell morphological examinations, and postoperative pathology, the patient was diagnosed with multiple myeloma, acute renal failure, hypercalcemia, hematopoietic dysfunction of bone marrow, and pulmonary infection. Moreover, SPECT revealed diffuse uptake of ⁹⁹Tc^m-MDP in bilateral pulmonary. Relevant literature suggests that multiple myeloma may cause renal insufficiency and induce secondary hyperparathyroidism, thereby leading to hypercalcemia. In the resulting hypercalcemia, Ca²⁺ is passively transported along the concentration gradient to the pulmonary alveoli and bronchial epithelial cells, where Ca²⁺ binds to the mitochondria and forms metastatic microcalcifications that take up ⁹⁹Tc^m-MDP and are opacified in SPECT. Combining medical history, examination results, and relevant literature, we suspected that extensive metastatic microcalcium deposition in the bilateral pulmonary mainly causes the diffuse development of ⁹⁹Tc^m-MDP. The metastatic calcification in bilateral pulmonary is low; therefore, it is difficult to detect using common imaging methods. Moreover, these patients always have severe ventilatory disorders, which have extremely poor prognosis. Therefore, ⁹⁹Tc^m-MDP imaging may be used to evaluate the diagnosis and efficacy, thus suggesting that clinicians should perform active symptomatic treatment as early as possible to improve prognosis and reduce mortality.