In a case of osteogenesis imperfecta with multiple fractures already from childhood, myelomatosis was diagnosed at the age of 52 years because of a serum M-component (IgG, lambda), Bence Jones proteinuria, myeloma cells in the bone marrow, and osteolytic skeletal lesions. She died 10 months later. A partial postmortem examination of a larger bone lesion confirmed the diagnosis.