Synthetic oligodeoxynucleotides (ODNs), such as PF-3512676, that contain unmethylated cytosine-guanine motifs (CpG ODN) have been identified as highly potent immune activators by in vitro examinations and in murine models. CpG ODNs induce innate and adaptive immune responses by triggering Toll-like receptor 9 expressed by human B cells and plasmacytoid dendritic cells. A phase 1 study was initiated to investigate safety, tolerability, serum cytokine levels, cellular immune responses, and clinical activity of intralesional treatment with PF-3512676 in patients with basal cell carcinoma (BCC) or cutaneous or subcutaneous melanoma metastases. Intrapatient escalating doses of PF-3512676 (up to 10 mg) were injected intralesionally every 14 days in 5 patients with BCC and in cutaneous or subcutaneous metastases of 5 patients with melanoma. PF-3512676 was well tolerated. Local swelling and erythema occurred at the injection site in 9/10 patients. There was only 1 incidence of a grade III hematologic adverse event (lymphocytopenia). Local tumor regressions were observed in patients with BCC (1 complete regression, 4 partial regressions) and metastatic melanoma (1 complete regression). After treatment with PF-3512676, interleukin-6 was increased in all patients, interferon-gamma induced protein-10 in 8/10 patients, interleukin-12p40 in 7/10 patients, and tumor necrosis factor-alpha levels in 6/10 patients. All patients had biopsies; moderate to abundant cellular infiltrates of lymphocytes were found posttreatment in most lesions of both histologic types. Intralesional treatment of skin tumors with PF-3512676 was safe and well tolerated. Despite the relatively low dosage, clinical activity was demonstrated both in patients with BCC and with cutaneous or subcutaneous metastatic melanoma lesions.