BACKGROUND: The weight-loss effect of team medical care in which counseling is provided by clinical psychologists was investigated in an university hospital obesity (OB) clinic. Nutritional and exercise therapy were also studied. In our previous study, we conducted a randomized, controlled trial with obese patients and confirmed that subjects who received counseling lost significantly more weight than those in a non-counseling group. The purpose of this study was to identify the psychological characteristics assessed by ego states that promote behavior modification by obese patients. METHODS: 147 obese patients (116 females, 31 males; mean age: 45.9 +/- 15.4 years) participated in a 6-month weight-loss program in our OB clinic. Their psychosocial characteristics were assessed using the Tokyo University Egogram (TEG) before and after intervention. The Wilcoxon signed rank test was used to compare weight and psychological factors before and after intervention. Multiple regression analysis was used to identify factors affecting weight loss. RESULTS: Overall, 101 subjects (68.7%) completed the program, and their data was analyzed. The subjects mean weight loss was 6.2 +/- 7.3 kg (Z = 7.72, p < 0.01), and their mean BMI decreased by 2.4 +/- 2.7 kg/m2 (Z = 7.65, p < 0.01). Significant differences were observed for the Adult (A) ego state (0.68 +/- 3.56, Z = 1.95, p < 0.05) and the Free Child (FC) ego state (0.59 +/- 2.74, Z = 2.46, p < 0.01). The pre-FC ego state had a significant effect on weight loss (beta = 0.33, p < 0.01), and a tendency for changes in the A ego state scores to affect weight loss (beta = - 0.20, p = 0.06) was observed. CONCLUSION: This study of a 6-month weight-loss program that included counseling by clinical psychologists confirmed that the A ego state of obese patients, which is related to their self-monitoring skill, and the FC ego state of them, which is related to their autonomy, were increased. Furthermore, the negative aspects of the FC ego state related to optimistic and instinctive characteristics inhibited the behavior modification, while the A ego state represented objective self-monitoring skills that may have contributed to weight loss.