A Study to Customize Ibrutinib Treatment Regimens for Participants With Previously Untreated Chronic Lymphocytic Leukemia

The purpose of this study is to evaluate the efficacy and safety of ibrutinib + venetoclax (I+V) and ibrutinib monotherapy regimens in which dosing of ibrutinib is either proactively reduced or reactively modified in response to adverse events (AEs).

Inclusion Criteria: * Diagnosis of chronic lymphocytic leukemia/ small lymphocytic lymphoma (CLL/SLL) as per International Workshop on Chronic Lymphocytic Leukemia (iwCLL) 2018 diagnostic criteria * For ibruinib + venetocIax (I+V) cohorts: eastern cooperative oncology group (ECOG) performance status of 0-1. For ibrutinib monotherapy cohorts: ECOG performance status of 0-2 * Measurable nodal disease by computed tomography (CT), defined as at least 1 lymph node greater than and equal to (\>=) 1.5 centimeters (cm) in longest diameter * A participant using oral contraceptives must use an additional contraceptive method * A participant must agree not to be pregnant, breastfeeding, or planning to become pregnant while enrolled in this study or until 1 month after last dose or per local label if more conservative (for example, 3 months in European Union or Canada and 1 month in United States) Exclusion Criteria: * Uncontrolled autoimmune hemolytic anemia or idiopathic thrombocytopenia purpura, such as those participants with a declining hemoglobin level or platelet count secondary to autoimmune destruction within the 4 weeks prior to first dose of study treatment, or the need for prednisone greater than (\>) 20 milligrams (mg) daily (or corticosteroid equivalent) to treat or control the autoimmune disease * Known bleeding disorders (example, von Willebrand's disease or hemophilia) * Stroke or intracranial hemorrhage within 6 months prior to enrollment * Known or suspected Richter's transformation or central nervous system (CNS) involvement * Currently active, clinically significant cardiovascular disease, such as uncontrolled arrhythmia or Class II, III, or IV congestive heart failure as defined by the New York Heart Association Functional Classification