Background Mycosis fungoides and Sézary syndrome are a class of lymphomas of skin-trafficking T cells, and they are the most common forms of cutaneous T-cell lymphoma (CTCL). Mycosis fungoides and Sézary syndrome are chronic, frequently incurable diseases with limited therapeutic options. PF-3512676 (formerly CPG 7909) is a Toll-like receptor 9 agonist that is being investigated for treatment of patients with advanced cancer. Objective This study was conducted to determine the safety and tolerability of single-agent PF-3512676 in patients with CTCL. Methods In this phase I dose-escalation study, patients (N = 28) with treatment-refractory, stage IB to IVA CTCL were enrolled in 6 sequential cohorts and treated with PF-3512676 (0.08, 0.16, 0.24, 0.28, 0.32, or 0.36 mg/kg) administered as 24 weekly subcutaneous injections. Primary end points were safety and tolerability. Results Common adverse events (fatigue, rigors, injection-site reactions, myalgia, lymphopenia, leukopenia, neutropenia, and pyrexia) were mostly grade 1 or 2, and no patient developed specific symptoms associated with autoimmune disease. Clinical response rate to PF-3512676, as determined by both Composite Assessment of Index Lesion Severity and Physician Global Assessment, was 32% (3 complete clinical responses, 6 partial responses); the majority of responses (7/9; 78%) were ongoing at the end of study. Limitations This trial was not designed to rigorously assess efficacy. Conclusion Single-agent PF-3512676 was well tolerated and demonstrated antitumor activity in patients with refractory CTCL.