Background: The achievement of the same target for HbA1c had different results for cardiovascular (CV) outcome and mortality in some large-scale clinical trials. A similar phenomenon was reported for blood pressure (BP) and lipid levels. A possible reason may be that values were measured at different time points and frequencies. Differences may also have arisen due to consideration of seasonal variations. However, the monthly achievement rates for each value that are required to determine the specific treatment strategy remains unknown. In the present study, we evaluated, for the first time, rates of achievement of guideline targets for HbA1c, BP, and low-density lipoprotein (LDL)-cholesterol on a monthly basis in patients with type 2 diabetes mellitus (T2DM) and explored factors affecting the achievement rates. Methods: This retrospective study initially analyzed data on 104,601 patients with T2DM. Thirty-eight medical clinics or general/university-affiliated hospitals specializing in diabetes care volunteered to participate in this study. patients whose HbA1c, BP, and LDL-cholesterol were measured 12 or more times during the two-year period between January 2013 and December 2014 were included. We evaluated rates of achievement of guideline targets for HbA1c, BP, and LDL-cholesterol on a monthly basis. Achievements of targets were defined as HbA1c <7%, HbA1c <8%, SBP <130 mmHg, DBP <80 mmHg, and LDL-cholesterol <100 mg/dL according to the American Diabetes Association. The proportion of patients to achieve guideline targets for HbA1c, SBP, DBP and LDL-cholesterol was expressed as all ABC achievement rate. Summer season was defined as June, July, and August, winter season as December, January, and February. Furthermore, we used multiple logistic regression analysis to evaluate factors affecting the ABC achievement rates in the summer and winter. Findings: A total of 4,678 (2948 males and 1730 females) patients were finally analyzed. Mean age of participants was 61.3 ± 9.4 years (mean ± SD), mean duration of diabetes was 12.6 ± 8.6 years, and mean HbA1c was 57.2 ± 13.5 mmol/mol (mean % HbA1c 7.4 ± 1.2%). The ABC achievement rate and the achievement rates of HbA1c, BP, and LDL-cholesterol exhibited seasonal variations, and particularly low achievement rates in the winter season (all ABC achievement rate: summer 15.6%, winter 9.6%). In multiple logistic regression analysis, female gender was independently related to the increased achievement rate of SBP in the winter. In winter, advanced age (≥56 years) and diabetes duration <10 years were independently associated with a decrease in the achievement rate of SBP. Diabetes duration ≥5 years was independently associated with a decrease in the achievement rate of HbA1c in winter. There was no factor that associated with a lower achievement rate for LDL-cholesterol in winter. Interpretation The results of this study show that the state of patients' control of HbA1c, BP, and LDL-cholesterol completely changes depend on the timing of the evaluation of these parameters. Large-scale clinical trials should be conducted taking into account the seasonal variation of achievement rate for these parameters. A clinical practice that focuses on seasonal variation of achievement of targets may increase the year-round achievement rate and further reduce the CV events. Funding Statement: The authors report None. Declaration of Interests: The authors of this manuscript have the following competing interests: M.S. has participated in speaker’s bureaus/advisory panels for Sanofi, Daiichi-Sankyo, Astellas, and Tanabe-Mitsubishi. N.T. has received a research grant and support from Daiichi-Sankyo and Otsuka Pharmaceutical and has participated in speaker’s bureaus organized by Daiichi-Sankyo and MSD. The other authors have no conflict of interest to declare. Ethics Approval Statement: The ethics committee of the JDDM, which also included outside members, such as lawyers and ethics experts, approved the present study. The JDDM operates as an aggregate organization under the supervision of the central analytical facility and an ethics committee. All patients provided informed consent at each participating institute in accordance with the Guidelines for Epidemiological Study of the Ministry of Health, Labor and Welfare of Japan.