The usefulness of an educational package on hypertension that provides clinically important, up-to-date medical information and office "aids" to primary care physicians was tested in a randomized controlled trial. Fifty-six physicians completed a pretest multiple-choice questionnaire and were allocated at random either to a group that received the educational package (the "study group") or to a control group. There was a highly significant correlation between the pretest scores and the number of years since graduation (r = -0.55, p less than 0.0001), which indicated that younger physicians are more likely than older physicians to have an up-to-date knowledge of the management of hypertension. The increase in knowledge in the study group (17.5%) was significantly greater than that in the control group (2.7%). Furthermore, although the post-test scores in the control group were still significantly correlated with the number of years since graduation, those in the study group were not. It was concluded that although the older physicians knew less than their younger colleagues about hypertension, the use of the educational package significantly increased knowledge, and the increase was not limited by the physician's age.