Purpose: PF-3512676 (formerly CpG 7909) is a novel Toll-like receptor 9-activating oligonucleotide with single-agent antitumor activity that augments preclinical rituximab efficacy. This Phase I trial was designed to investigate the safety, tolerability, and preliminary antitumor activity of PF-3512676 in combination with rituximab. Experimental Design: Patients with relapsed/refractory CD20+ B cell non-Hodgkin's lymphoma received i.v. rituximab (375 mg/m2/week for 4 weeks) and PF-3512676 weekly for 4 weeks either i.v. (0.04, 0.16, 0.32, or 0.48 mg/kg) or s.c. (0.01, 0.04, 0.08, or 0.16 mg/kg). An additional extended-treatment cohort received 4 weeks of 0.24 mg/kg s.c. PF-3512676 in combination with rituximab followed by s.c. PF-3512676 alone weekly for 20 weeks. Results: Patients (N = 50) had received a median of three prior therapies (range, 1-11) including rituximab in 80% of patients. Treatment-related adverse events occurred in 11 of 19 (58%) i.v. patients, 15 of 19 (79%) s.c. patients, and all 12 patients in the extended-treatment cohort. Most common adverse events were mild to moderate systemic flu-like symptoms and injection-site reactions (s.c. cohorts only). Grade 3/4 neutropenia occurred in four patients. Objective responses occurred in 12 of 50 (24%) patients overall and in 6 of 12 (50%) patients in the extended-treatment cohort, including 2 patients with rituximab-refractory disease. Conclusion: Brief or extended-duration PF-3512676 can be safely administered in combination with rituximab in patients with relapsed/refractory non-Hodgkin's lymphoma.