INTRODUCTION: The combined effects of energy intake (EI) and physical activity (PA) on obesity have been poorly investigated. We have investigated the combined effects of EI and PA quantitatively in Japanese men and women with type 2 diabetes. METHODS: Data on 1395 patients with type 2 diabetes who attended 25 diabetes clinics located throughout Japan, obtained by questionnaire, were analyzed. A logistic regression model was used to calculate the odds ratio for obesity. RESULTS: Multi-adjusted odds ratios for the top versus the bottom tertile of EI and the bottom versus the top tertile of PA were 1.39 (95% confidence interval [CI] 1.02-1.89) and 1.64 (95% CI 1.22-2.22), respectively. The combination of EI (kcal/day) ≥ 1967 and PA (metabolic equivalents [METs] h/week) ≤ 9.9 for men and of EI ≥ 1815 and PA ≤ 8.3 for women were significantly associated with obesity. CONCLUSIONS: The existence of "allowable maximum EI" and "required minimum PA" that is significantly associated with "not being obese" is implied, suggesting the need for lifestyle education for Japanese patients with type 2 diabetes.