The bone marrow is where the blood cells are made.  A bone marrow biopsy involves taking a sample of bone marrow, usually from the back of the iliac crest (hip bone) or from the sternum (breastbone) to see how well the bone marrow is functioning, and if there are any abnormal cells in the bone marrow.  The procedure is performed in the Haematology Day Stay ward.  Nitrous oxide gas (Entonox) is used for pain relief.  Local anaesthetic is also given as an injection under the skin to numb the skin and surface of the bone.  You may have some soreness and bruising at the biopsy site afterwards, and may need to use paracetamol or other mild painkiller for a few days.