To describe internal medicine residents' opinions regarding the optimal duration of internal medicine residency training, and to assess whether these opinions are associated with specific career interests. A national cohort study was conducted during the 2005 Internal Medicine In-Training Examination (IM-ITE), which involved 382 of 388 (98.5%) U.S. internal medicine programs. A sample of 14,579 residents enrolled in three-year categorical or primary care training programs in the United States reported their opinions regarding optimal residency training duration on the IM-ITE 2005 Residents Questionnaire. Reported optimal training duration was assessed by postgraduate training year, sex, medical school location, program type, and reported career plan. Among the residents surveyed, 78.1% reported a three-year optimal length of internal medicine residency training, 15.3% preferred a two-year training duration, and 6.7% preferred a four-year duration. Residents planning careers in general medicine, hospital medicine, and subspecialty fields all preferred a three-year training duration (83.8%, 82.6%, and 75.9%, respectively). Residents planning subspecialty careers were more likely than those planning general or hospital medicine careers to prefer a two-year program (18.7% versus 7.4% and 8.3%). Residents planning generalist or hospitalist careers were more likely to favor a four-year program (8.9% and 9.1%, respectively) compared with residents planning subspecialty careers (5.4%). Most internal medicine residents endorse a three-year optimal duration of internal medicine residency training. This perspective should be considered in further national discussions regarding the optimal duration of internal medicine training.