Background: Guidelines recommend initiating metformin at the time of T2D diagnosis in the absence of contraindications for asymptomatic patients. However, the evidence is scarce regarding real-world patterns and factors associated with antidiabetic treatment initiation in patients with newly diagnosed T2D. Methods: We used a 5% random sample of Medicare beneficiaries newly diagnosed with T2D in 2007-2017 and followed them for one year. The outcome was time to initiation of an antidiabetic agent within the first year of diagnosis. We examined trends in the proportion of outcome and the therapeutic class initiated by year. We constructed Cox Proportional Hazard models to identify factors associated with treatment initiation. Results: Of 231,408 patients newly diagnosed with T2D, the mean age was 71.7, and 62.2% were female. Only 13.7% initiated an antidiabetic agent within the first year of diagnosis. Although the proportion of initiation within one year of T2D diagnosis was relatively constant over time (12.7% in 2007 to 15.4% in 2017), the mean time to initiation decreased from 74.1 days in 2007 to 21.7 days in 2017 (p<0.05). Of those who initiated treatment, 54.2% and 21.5% initiated metformin and sulfonylurea in 2007, while 84.4% and 6.0% initiated metformin and sulfonylurea in 2017, respectively. In the adjusted model, age (HR 0.92, 95%CI 0.91-0.93 for 10-year increase), female sex (HR 0.89, 95%CI 0.87-0.91), number of comorbidities (HR 0.86, 95%CI 0.86-0.87 for one additional) and residence in urban areas (HR 0.78, 95%CI 0.76-0.80) were associated with decreased hazards of antidiabetic initiation. Conclusion: We found unexpectedly low rates of initiation of first-line treatment among Medicare beneficiaries with newly diagnosed T2D. The changes in the proportion of different drug classes reflect the change in guideline recommendations. Further investigation is needed to understand the causes underlying the low initiation. Disclosure Y. Li: None. I. Hernandez: Consultant; Self; Bristol-Myers Squibb Company, Pfizer Inc. N. Gabriel: None. S. L. Kane-gill: None. U. Essien: None. F. Toledo: Consultant; Self; AstraZeneca. J. Guo: None.