We have previously shown that oligonucleotides designed to bind in triplex fashion to a specific p53 binding site homology inhibit the proliferation of colon cancer cells in vitro. The present study was designed to extend these observations in an in vivo model. HCT 116 human colon carcinoma cells were injected subcutaneously into Ncr nude mice and tumors formed at one to two weeks. Tumors were injected daily for 14 days with either triplex forming oligonucleotide (Hoog 1), a scrambled Hoog 1 oligonucleotide (Hoog3) as control, or vehicle. Tumor size was measured twice weekly. Active triplex forming oligonucleotide (Hoog1) reduced tumor size in comparison to either control oligonucleotide (Hoog3) or vehicle. Tumor sizes in the three groups were significantly different (P < 0.001). Student Newman Keuls test shows statistically significant differences between the experimental group and each of the control and vehicle groups (P < 0.05). A triplex forming oligonucleotide directed at a p53 consensus binding site reduces tumor growth suggesting a novel method of tumor inhibition.