Medicinal plants and herbs are widely used in the treatment of various types of cancer in Taiwan, China and many other countries. Hedyotis diffusa Willd (HDW) has been known as a traditional Chinese medicine for a long time, and possesses various bioactivities and anticancer activity. There is no available information on the effects of HDW extracts in leukemic mice and on immune responses in vivo. In this study, we established murine WEHI-3 leukemia in BALB/c mice and hypothesized that an aqueous HDW extract might have antileukemia effects on leukemic animals in vivo. The major characteristic of leukemic mice was an enlarged spleen after intraperitoneal injection with WEHI-3 cells. HDW extract reduced the weights of spleen and liver, but had no significant effect on body weight in WEHI-3 leukemic mice. HDW extract increased the percentage of CD11b cell surface marker (monocytes), but it reduced the percentage of CD3 (T-cell) and CD19 (B-cell) markers. However, HDW extract did not affect the level of Mac-3 and there was no influence on phagocytosis by macrophages from peripheral blood mononuclear cells and the peritoneal cavity in leukemic mice. The isolated splenocytes from HDW extract-treated leukemic mice demonstrated an increase of T- and B-cell proliferation in vivo. Based on these results, HDW extract would appear to have antileukemia activity in WEHI-3 cell-induced leukemia in vivo.