Dr. Riddle attended Yale University, receiving his BA magna cum laude, and received his medical degree from Harvard Medical School. He performed his internship and residency in internal medicine at Rush-Presbyterian-St.Luke's Medical Center in Chicago, and completed two fellowships in Endocrinology and Metabolism, one at Rush Presbyterian-St.Luke's with Dr. Theodore Schwartz (1969-71), and the other at the University of Washington in Seattle with Dr. Robert Williams (1971-73). Since 1973 he has been at Oregon Health & Science University, where he is Professor of Medicine in the Division of Endocrinology, Diabetes, & Clinical Nutrition.
His interests include the physiology of type 2 diabetes, basal insulin therapy, newer therapeutic agents, combination therapy, cardiovascular outcomes of diabetes, and the organization of care for diabetes. He has authored more than 210 publications, is an Associate Editor of Diabetes Care, and has served on the editorial boards of Diabetes Technology & Therapeutics and the Journal of Clinical Endocrinology & Metabolism. Among other activities in clinical research he has leadership responsibilities in various large international trials, including ACCORD/ACCORDION, ORIGIN/ORIGINALE, ELIXA, EXSCEL, REWIND, and DCCT/EDIC, all of which are testing the effects of treatments on cardiovascular and other medical outcomes in diabetes. The American Diabetes Association gave him their Outstanding Physician-Clinician Award in 2005.