Introduction: Venetoclax (V) / Obinutuzumab (O) is an effective one-year fixed duration therapy for patients (pts) requiring first-line CLL treatment (tx). In CLL14, tumor lysis syndrome (TLS) risk assessment (including lymph node measurements by imaging and ALC) was required once prior to tx initiation, and O's impact on TLS risk prior to introduction of V was not a pre-planned study endpoint. Therefore, TLS prophylaxis (ppx) and tx setting for V dose escalation did not account for the possibility that O may decrease TLS risk prior to V escalation. Based on this approach, 22% of pts in the CLL14 study were high risk for TLS, thus requiring hospitalization for intensive laboratory monitoring during V dose escalation. (Fischer, NEJM 2019) We hypothesized that cycle (C) 1 of O would significantly decrease TLS risk prior to V escalation and that all pts, regardless of TLS risk category, could safely undergo a standard 5-week V dose escalation in the outpatient setting.