Dr. Ahnen completed his Medical School Training at Wayne State University in Detroit Michigan and completed a Medical Residency and Chief Residency at Hutzel Hospital in Detroit before going to the University of Colorado as a Fellow in Gastroenterology in 1977. After completing his Fellowship, he completed a Membrane Pathobiology Research Fellowship at Stanford University then returned to join the faculty of the University of Colorado School of Medicine. In 2014, he retired from his long-standing (32 years) faculty position at The Department of Veterans Affairs Eastern Colorado Health Care System and in 2016, he retired from the University of Colorado School of Medicine faculty as Professor of Medicine. In 2017, he was appointed Professor Emeritus at the University of Colorado School of Medicine. After his retirement, he joined the staff of Gastroenterology of the Rockies in Boulder/Denver CO.

Dr. Ahnen has been an active clinician and investigator throughout his career. He was the Co-Founder and Co-Director of the University of Colorado Hereditary Cancer Clinic until 2015 when he founded the Genetics Clinic at Gastroenterology of the Rockies where he provides consultative service to GI cancer families. His laboratory has conducted both basic and clinical investigations related to colorectal cancer prevention. Dr. Ahnen’s basic science laboratory examined the biologic and biochemical mechanisms of the chemopreventive effects of the non-steroidal anti-inflammatory drugs (NSAIDs). Dr. Ahnen’s clinical laboratory group conducted colorectal cancer chemoprevention and screening trials.  His behavioral laboratory group conducted trials designed to improve colorectal cancer screening rates in high-risk populations.

Dr. Ahnen is currently working to implement colorectal cancer prevention interventions that have been shown to be effective in clinical trials seamlessly into a community-based practice, Gastroenterology of the Rockies.