Increased CD4(+)CD25(+) regulatory T cells predicted poor prognosis in ovarian carcinoma patients. This study aimed to define whether soluble substances secreted by ovarian carcinoma could up-regulate the proportion of CD4(+)CD25(+) regulatory T cells. Similar to TGF-beta, the low MWF (<50kDa) of supernatant derived from SKOV3 could convert part of freshly isolated CD4(+)CD25(-) T cells into CD25(+) population with similar characters as natural CD4(+)CD25(+) regulatory T cells. To deplete TGF-beta in the low MWF by neutralizing anti-TGF-beta would eliminate this transformation phenomenon. These results indicate that TGF-beta secreted by ovarian carcinoma cells owns vital function in the process of converting peripheral CD4(+)CD25(-) T cells into CD4(+)CD25(+) regulatory T cells, which may provide one immunotherapeutic target for ovarian cancer.