Bone marrow is a soft, fatty tissue inside the bones where blood cells (red blood cells, platelets, and white blood cells) are produced and developed. Bone marrow biopsies (collection of a small sample for examination) are used to evaluate blood disorders and diagnosis certain types of cancers. They can also be helpful in diagnosing fevers of unknown origins. A bone marrow biopsy can be an aspiration, a needle biopsy, or both.