Dissemination of new medical information to the practicing physician is a complex and often faulty process. To examine the magnitude of this problem, we surveyed primary care physicians to determine their knowledge of the results of the cooperative trial of photocoagulation in diabetic retinopathy. Despite the acknowledge relevance to their practice, only 28% (38/137) of family physicians and 46% (42/91) of internists were aware of the study results (P less than .001). Respondents were asked to manage two patient problems involving diabetic retinopathy. Only 33% (75/229) handled both correctly, although the retinopathy photocoagulation study had been published 18 months earlier. These findings indicate that results from clinical trials may not be disseminated to practicing physicians and, therefore, not incorporated into practice. Greater attention should be directed toward making findings from clinical trials available to practitioners.