To investigate the antitumor action of arsenic trioxide (As2O3) by intratumoral injection into solid tumors, tumor growth inhibition (TGI) and angiogenesis of heterotransplanted esophageal carcinoma in mice was carried out. The cultured human esophageal carcinoma cells were inoculated into both laterals of the abdominal wall of severe combined immunodeficient (SCID) mice. When both lateral tumors had grown to about 10x8x5 mm(3), the right tumors were treated with an intratumoral injection of As2O3 in dosage of 1, 5 and 10 microg per day, respectively, for 10 days sequentially. Left tumors were treated with PBS (phosphate buffer solution) as control. The weight of transplanted tumor masses were measured and counted for TGI. The tissue of tumor, liver, kidney, heart, lung and brain was examined histopathologically and tumor tissues were examined by light- or electron-microscope. Ki-67 and CD34 were assessed by immunohistochemistry and positive nuclei of Ki-67 and microvessel density (MVD) labeled by CD34 were measured. The results revealed that on the 20th day after the first injection, As2O3-treated tumors were suppressed markedly as compared with the contrarily situated tumor, accompanied by a marked apoptosis and necrosis in tumor cells. The tissue of liver, kidney, heart, lung and brain was unaffected by As2O3. MVD in tumor tissue was decreased in the right side tumor with the significant difference in the 5 micro g and 10 micro g group (p<0.01). TGI was 5.80 (p>0.05), 58.66 (p<0.01) and 73.97% (p<0.01) in the 1, 5 and 10 micro g groups respectively, but 2.21% (p>0.05) in the control group. Conclusively, a repeated administration of As2O3 (5 and 10 microg x 10) induced an increase of tumor growth inhibition and decrease of angiogenesis in the solid tumor in tumor progressive periods. These results suggest that intra-tumoral injection of As2O3 may be investigated as a modality to treat some solid tumors.