A diabetic patient who was misdiagnosed as rheumatoid arthritis because of complicated musculoskeletal manifestations of diabetes is reported here. A 57 year old woman had been on sulphasalazine treatment with a diagnosis of rheumatoid arthritis for 3 years but failed to respond. Her past medical history disclosed that she had been using metformin due to diabetes mellitus for 8 years. On physical examination there was no evidence of arthritis. Her clinical picture was compatible with diffuse idiopathic skeletal hyperostosis (DISH), shoulder periarthritis, carpal tunnel syndrome, limited joint mobility of diabetes and furthermore myalgia due to metformin induced by hypovitaminosis D. Finally rheumatoid arthritis was excluded and a diagnosis of diabetes mellitus originated diffuse musculoskeletal system involvement was made. Diabetic musculoskeletal complications are common and sometimes cause clinical dilemmas. This case is also important for highlighting the contribution of low vitamin D status to the clinical status.