 Rationale: Two Food and Drug Administration drugs approved for blood cancers such as non-Hodgkins lymphoma NHL include fludarabine Fludara and cladribine Leustat. The drug offered in the current study, clofarabine was designed to combine the anti-cancer strength of both fludarabine and cladribine. Laboratory research suggests that clofarabine targets anti-cancer mechanisms in cells, helps repair DNA, and inhibits tumor growth. Research also indicates that clofarabine has some efficacy against a variety of blood cancers and solid tumors. Numerous tumor responses have been observed with high doses of clofarabine in heavily pretreated patients with different types of lymphoma. The current study build on this previous research to test clofarabine in patients with aggressive NHL. 