Background. Medical schools are placing increased emphasis on training students in the ambulatory setting, but few studies show the benefit or academic risk of such innovation. The authors studied the effect of such a new rotation with a rigorous study design. Method. From a group of 166 third-year students at the Uniformed Services University of the Health Sciences F. Edward Hebert School of Medicine assigned in 1990-91 to do six weeks of their third-year medicine clerkship at Walter Reed Army Medical Center, 106 volunteers were randomized to six weeks either on the usual rotation on the general medicine wards (69 students) or to a new ambulatory rotation (37 students). Multiple pre-and postclerkship parameters were used to evaluate clerkship skills and knowledge; eventual internship choice was determined. Results. The randomization was successful. Postclerkship performances were the same in (1) the medicine subject examination of the National Board of Medical Examiners, (2) a multiple-choice test in interpreting laboratory results, (3) blinded rating by an expert panel of the quality of final written histories and physical exams, (4) blinded rating by an expert panel of the students’ written case analyses of their own patients, and (5) a written multiple-step examination of problem-solving ability. Any differences between groups favored the ambulatory group. An increase in choice of primary care internships among students randomized to the ambulatory rotation was not significant. Conclusion. It is possible to study innovative rotations using a prospective, randomized design. Substituting a six-week block ambulatory experience for a ward rotation did not decrease students’ abilities to write up or analyze complex cases.