Banaba in the Tagalog name, Lagerstroemia speciosa L., has been used as a folk medicine for a long time among diabetics in the Philippines. Extracts from banaba leaves have been reported to reduce diabetic symptoms in genetically diabetic mice (Type II, KK-Ay). In the present study, female mice of the same strain showing remarkable body weight gain were used to examine the antiobesity effect of dietary banaba extract. Five-week-old female KK-Ay mice were fed a control diet or test diet containing 5% of a hot-water extract from banaba leaves instead of cellulose for 12 wk. Neither group showed any changes in diet intake during the experimental period. Body weight gain and parametrial adipose tissue weight were lowered significantly in the banaba diet group. Blood glucose levels were not suppressed in the banaba diet group, but hemoglobin A1C was found to be suppressed at the end of the experiment. No effects on the serum lipids were observed, but the mice fed banaba extract showed a significant decrease, to 65% of the control level in total hepatic lipid contents. This decrease was due to a reduction in the accumulation of triglyceride. These results suggest that banaba had a beneficial effect on obese female KK-Ay mice.