The purpose of this study is to evaluate the short-term effectiveness of a weekly dietary instruction program to reduce obesity in elementary school children. A total of 140 third- to sixth-grade overweight children and 130 height- and age-matched overweight children were selected from two elementary schools in Kaohsiung. The primary inclusion criterion was a Rohrer's Index [RI = weight (kg)/height (cm)3 x 10(7)] greater than 150. The anthropometric measurement was performed on both the group receiving instruction and the group not receiving instruction. Biomedical monitoring of the students state of health, assessments of their nutritional knowledge, attitude and practice (K.A.P.) before and after the thirteen-week instruction were compared in the instruction group. After completion of the instruction program, a significant reduction of RI values (177.2 +/- 18.3 vs. 169.4 +/- 20.1, p < 0.01), and a significant increase in body height (144.9 +/- 7.7 cm vs. 146.7 +/- 7.7 cm, p < 0.01), with no change in body weight (54.5 +/- 11.3 kg vs. 54.3 +/- 11.3 kg) were found in the group receiving instruction. Some of the anthropometric parameters of obesity were also significantly improved. Reduction of total cholesterol and serum Glutamic Oxaloacetic Transaminase (GOT)/Glutamic Pyrvic Transaminase (GPT) levels were also observed; the later phenomena were interpreted as an improvement in the fatty metamorphosis of the liver, which usually accompanies obesity. A significantly increased nutrition knowledge and change to more healthy dietary behavior were also found in students who finished the instruction program. This reduction was a measure of the effectiveness of weekly dietary instruction sessions. An increase in NSQ scores indicated improved nutritional knowledge and behavior. Through the implementation of weekly nutritional instruction, a decrease in RI can be achieved in a thirteen-week period. We propose that frequent nutritional education should be part of the curriculum of elementary school children in order to prevent obesity-related diseases.