Introduction: Evaluation of total energy expenditure (TEE) and physical activity level (PAL) is important for treatment of patients with type 2 diabetes mellitus (T2DM). However, the validity of accelerometers (ACC) and physical activity questionnaires (PAQ) for estimating TEE and PAL remains unknown in elderly populations with T2DM. We evaluated the accuracy of TEE and PAL results estimated by an ACC (TEE ACC and PAL ACC ) and a PAQ (TEE PAQ and PAL PAQ ) in elderly patients with T2DM. Methods: Fifty-one elderly patients with T2DM (aged 61-79 years) participated in this study. TEE ACC was calculated with PAL ACC using a triaxial ACC (Active style Pro HJA-750c) over 2 weeks and predicted basal metabolic rate (BMR) by Ganpule's equation. TEE PAQ was estimated using predicted BMR and the PAL PAQ from the Japan Public Health Center Study-Long questionnaire. We compared the results to TEE DLW measured with the doubly labeled water (DLW) method and PAL DLW calculated with BMR using indirect calorimetry. Results: TEE DLW was 2,165 ± 365 kcal/day, and TEE ACC was 2,014 ± 339 kcal/day; TEE ACC was strongly correlated with TEE DLW (r = 0.87, p < 0.01) but Physical Activity Estimated by Triaxial ACC and Questionnaire 63 Ann Nutr Metab 2020;76:62-72 significantly underestimated (-150 ± 183 kcal/day, p < 0.05). There was no significant difference in TEE PAQ and TEE DLW (-49 ± 284 kcal/day), while the range of difference seemed to be larger than TEE ACC . PAL DLW , PAL ACC , and PAL PAQ were calculated to be 1.71 ± 0.17, 1.69 ± 0.16, and 1.78 ± 0.24, respectively. PAL ACC was strongly correlated with PAL DLW (r = 0.71, p < 0.01), and there was no significant difference between the 2 values. PAL PAQ was moderately correlated with PAL DLW (r = 0.43, p < 0.01) but significantly overestimated. Predicted BMR was significantly lower than the BMR measured by indirect calorimetry (1,193 ± 186 vs. 1,262 ± 155 kcal/day, p < 0.01). Conclusions: The present ACC and questionnaire showed acceptable correlation of TEE and PAL compared with DLW method in elderly patients with T2DM. Systematic errors in estimating TEE may be improved by the better equation for predicting BMR.