BACKGROUND Data regarding the efficacy of treatment with ibrutinib–rituximab, as compared with standard chemoimmunotherapy with fludarabine, cyclophosphamide, and rituximab, in patients with previously untreated chronic lymphocytic leukemia (CLL) have been limited. METHODS In a phase 3 trial, we randomly assigned (in a 2:1 ratio) patients 70 years of age or younger with previously untreated CLL to receive either ibrutinib and rituximab for six cycles (after a single cycle of ibrutinib alone), followed by ibrutinib until disease progression, or six cycles of chemoimmunotherapy with fludarabine, cyclophosphamide, and rituximab. The primary end point was progression-free survival, and overall survival was a secondary end point. We report the results of a planned interim analysis. RESULTS A total of 529 patients underwent randomization (354 patients to the ibrutinib–rituximab group, and 175 to the chemoimmunotherapy group). At a median follow-up of 33.6 months, the results of the analysis of progression-free survival favored ibrutinib–rituximab over chemoimmunotherapy (89.4% vs. 72.9% at 3 years; hazard ratio for progression or death, 0.35; 95% confidence interval [CI], 0.22 to 0.56; P<0.001), and the results met the protocol-defined efficacy threshold for the interim analysis. The results of the analysis of overall survival also favored ibrutinib–rituximab over chemoimmunotherapy (98.8% vs. 91.5% at 3 years; hazard ratio for death, 0.17; 95% CI, 0.05 to 0.54; P<0.001). In a subgroup analysis involving patients without immunoglobulin heavy-chain variable region (IGHV) mutation, ibrutinib–rituximab resulted in better progression-free survival than chemoimmunotherapy (90.7% vs. 62.5% at 3 years; hazard ratio for progression or death, 0.26; 95% CI, 0.14 to 0.50). The 3-year progression-free survival among patients with IGHV mutation was 87.7% in the ibrutinib–rituxi-mab group and 88.0% in the chemoimmunotherapy group (hazard ratio for progression or death, 0.44; 95% CI, 0.14 to 1.36). The incidence of adverse events of grade 3 or higher (regardless of attribution) was similar in the two groups (in 282 of 352 patients [80.1%] who received ibrutinib–rituximab and in 126 of 158 [79.7%] who received chemoimmunotherapy), whereas infectious complications of grade 3 or higher were less common with ibrutinib–rituximab than with chemoimmunotherapy (in 37 patients [10.5%] vs. 32 [20.3%], P<0.001). CONCLUSIONS The ibrutinib–rituximab regimen resulted in progression-free survival and overall survival that were superior to those with a standard chemoimmunotherapy regimen among patients 70 years of age or younger with previously untreated CLL.