Based solely on clinical clues from a malnourished population, thiamin alone was intentionally and successfully injected to human cases with some tumors or masses. Two cases of submandibular gland cyst and 13 out of 15 cases of Baker's cyst were cured without recurrence for several decades. In a case with pathology-confirmed osteosarcoma, subcutaneous perfusion of thiamin HCl 300 once only reduced its circumference from 30 to 20 cm, equivalent to a reduction of 50-75% in volume, within 2 days. Current concepts on the role of thiamin in carcinogenesis are controversial. Some authors claimed that thiamin supported high rate of tumor cell survival, proliferation and chemotherapy resistance and suggested anti-thiamin therapy for cancer. On the other hand, some investigators have reported evidence of prevention of several varieties of cancers by dietary thiamin. A limited number of animal studies revealed evident relationship between thiamin deficiency and cancer development. Therefore, further study on the mechanism switching thiamin between cancer supporter and suppressor is needed.