The NHLBI has recently developed standards for protection of subject confidentiality which enable distribution of large data sets collected in completed epidemiological studies and clinical trials. This allows other investigators to pursue research using the data these studies have generated. An ethical commitment to protect the privacy of the research subjects requires careful review and some compromise in data details prior to distribution. To meet the challenge posed by the ethical and legal requirements to protect participants' privacy and making data sets useful for researchers, the NHLBI has devised a policy which seeks to balance these two opposing forces. This paper describes this policy as well as the limited access data sets currently available from NHLBI.