High bone mineral density (BMD) is common and sometimes an incidental finding. The causes are numerous. Among them, none has previously been attributed to total body irradiation (TBI). We present the case of a 56-year-old female patient with a history of T-lymphoblastic lymphoma at age 33 who was treated with allogeneic hematopoietic stem cell transplantation following a conditioning regimen including a single-fraction 10 Gray TBI. This patient was in complete remission but experienced several transplant-related late effects. She presented to the rheumatology outpatient clinic with chronic mechanical low back pain and a history of early menopause. Bone assessment by densitometry revealed high bone mineral density with a lumbar spine L2-L4 T-score of +6.3 standard deviation (SD) (1.939 g/cm²), right femoral neck T-score of +7.2 SD (1.849 g/cm²), right total femur T-score of +4 SD (1.484 g/cm²), distal radioulnar T-score of +0.7 SD (0.495 g/cm²). Imaging revealed sclerotic lesions in the vertebrae, femoral cortices and pelvis. An etiological workup excluded other causes such as fluorosis, mastocytosis, renal osteodystrophy, hypoparathyroidism/pseudohypoparathyroidism and myelofibrosis. Bone growth factors and resorption markers were normal. Genetic sequencing showed no significant abnormalities. Based on this comprehensive evaluation, TBI was identified as a possible contributing factor to the occurrence of high BMD. The patient was managed with analgesics and regular follow-up. This case highlights the importance of a systematic etiologic approach to high bone mineral density and underscores the need for future scientific research to better understand this phenomenon for which the pathological relationship with radiation exposure remains unknown.