We report a case of secondary osteoporosis treated with a combination of vitamins D3 and K2, administered orally. A 13-year-old male, diagnosed with highly differentiated acute myelogenous leukaemia, received an allogeneic bone-marrow transplantation. Chronic graft-versus-host disease persisted, thereafter, in the form of severe diarrhoea, rash and allergic conjunctivitis. Since the patient was then at risk from osteoporosis secondary to calcium malabsorption caused by the diarrhoea, dual-energy X-ray absorptiometry and ultrasound analysis were used to measure bone mineral density and bone stiffness, respectively. Both measurements were markedly lower than the average values from patients of matched age, gender and physical characteristics. The osteoporosis did not respond to active vitamin D3 0.1 microg/kg once daily, but when this therapy was combined with vitamin K2 15 mg once daily, an increase in bone mineral density and bone stiffness was observed. In conclusion, vitamin D3 and K2 combination therapy merits further evaluation for the treatment of various types of secondary osteoporosis, including steroid-induced osteoporosis.