A Study to Evaluate the Efficacy of Venetoclax Monotherapy in Relapsed/Refractory Participants With Chronic Lymphocytic Leukemia (CLL)

* Participant has Eastern Cooperative Oncology Group (ECOG) performance score of less than or equal to 2 * Participant has relapsed/refractory disease (received at least 1 prior therapy) * Participant has diagnosis of CLL that meets published 2008 Modified International Workshop for Chronic Lymphocytic Leukemia National Cancer Institute Working Group (IWCLL NCI-WG) Guidelines and: * has an indication for treatment according to the 2008 Modified IWCLL NCI-WG criteria * has clinically measurable disease (lymphocytosis greater than 5 × 10\^9/L and/or palpable and measurable nodes by physical exam and/or organomegaly assessed by physical exam) * In addition, participants: * with or without 17p deletion or TP53 mutation, assessed by a local laboratory in bone marrow or peripheral blood are eligible * may have been previously treated with a prior B-cell receptor inhibitor * Participant must have adequate bone marrow function, coagulation profile, renal, and hepatic function, per laboratory at Screening

* Participant has developed Richter's transformation or Prolymphocytic leukemia * Participant has previously received venetoclax * History of active malignancies other than CLL within the past 2 years prior to first dose of venetoclax, with the exception of: * adequately treated in situ carcinoma of the cervix uteri * adequately treated basal cell carcinoma or localized squamous cell carcinoma of the skin * previous malignancy confined and surgically resected (or treated with other modalities) with curative intent * Participant has active and uncontrolled autoimmune cytopenias (for 2 weeks prior to Screening), including autoimmune hemolytic anemia and idiopathic thrombocytopenic purpura despite low dose corticosteroids * Participant has undergone an allogeneic stem cell transplant * Treatment with any of the following within five half-lives or 14 days (if half-life unknown) as applicable prior to the first dose of venetoclax, or clinically significant adverse effect(s)/toxicity(s) of the previous therapy have not resolved to \< National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) v4.03 Grade 2: * Any anti-cancer therapy including chemotherapy, or radiotherapy; * Investigational therapy, including targeted small molecule agents * Participant is human immunodeficiency virus (HIV) positive * Participant has known allergy to both xanthine oxidase inhibitors and rasburicase