Tumor necrosis factor-α (TNF-α) is a potent cytokine with anticancer efficacy that can significantly enhance hyperthermic injury. However, TNF-α is systemically toxic, thereby creating a need for its selective tumor delivery. We used a newly developed nanoparticle delivery system consisting of 33-nm polyethylene glycol - coated colloidal gold nanoparticles (PT-cAu-TNF-α) with incorporated TNF-α payload (several hundred TNF-α molecules per nanoparticle) to maximize tumor damage and minimize systemic exposure to TNF-α. SCK mammary carcinomas grown in A/J mice were treated with 125 or 250 μg/kg PT-cAu-TNF-α alone or followed by local heating at 42.5°C using a water bath for 60 minutes, 4 hours after nanoparticle injection. Increases in tumor growth delay were observed for both PT-cAu-TNF-α alone and heat alone, although the most dramatic effect was found in the combination treatment. Tumor blood flow was significantly suppressed 4 hours after an i.v. injection of free TNF-α or PT-cAu-TNF-α. Tumor perfusion, imaged by contrast enhanced ultrasonography, on days 1 and 5 after treatment revealed perfusion defects after the injection of PT-cAu-TNF-α alone and, in many regions, complete flow inhibition in tumors treated with combination treatment. The combination treatment of SCK tumors in vivo reduced the in vivo/in vitro tumor cell survival to 0.05% immediately following heating and to 0.005% at 18 hours after heating, suggesting vascular damage - mediated tumor cell killing. Thermally induced tumor growth delay was enhanced by pretreatment with TNF-α-coated gold nanoparticles when given i.v. at the proper dosage and timing.