Early type 2 diabetes mellitus (DM) may only require lifestyle modifications for glycemic control without the need for oral hypoglycemic agents (OHAs). Metformin is believed to improve cardiovascular outcomes in patients with DM, and it is considered to be a first-line therapy. However, it is unclear whether metformin is beneficial for patients with a new diagnosis of DM compared to those who do not need OHAs for glycemic control.Data were obtained from a population-based health care database in Taiwan. Patients with a new diagnosis of DM were enrolled if they received metformin monotherapy only between 1999 and 2010. A 4:1 propensity score-matched cohort of patients with a new diagnosis of DM who did not take OHAs or insulin during follow-up was also enrolled. The primary study endpoint was the occurrence of major adverse cardiovascular events (MACEs). The time to the endpoints was compared between groups using Cox proportional hazards models.A total of 474,410 patients with DM were enrolled. During a mean 5.8 years of follow-up, the incidence of MACEs was 1.072% (1072 per 100,000 person-years) in the metformin monotherapy group versus 1.165% in the lifestyle modification group (those who did not take OHAs) (P < .001). After adjusting for confounders, metformin independently protected the DM patients from MACEs (hazard ratio: 0.83, P < .001). The metformin group also had an improved MACE-free survival profile from year 1 to year 12 (P < .001).In addition to lifestyle modifications, the patients with a new diagnosis of DM treated with metformin monotherapy had a lower MACE rate than those who did not take OHAs. Our findings suggest that metformin may be given early to patients with a new diagnosis of DM, even when they do not need OHAs for glycemic control.