The call for a "parallel track" for AIDS drug developmenta proposal that would allow the early distribution of AIDS drugs to large numbers of patients in parallel with the conventional clinical trials that assess the drugs’ safety and efficacyhas sparked controversy within the scientific community. Questions have arisen about the risks to patients of such a plan, about its potential effect on the successful completion of standard controlled trials, and about whether the parallel track will generate useful data.Larger questions have also been raised about whether the parallel track heralds fundamental changes in the philosophy underlying drug regulation in the United States, about the costs and financing of investigational therapies and associated medical costs, and about the role of expanded access mechanisms for drugs in reaching those whose health care is generally inadequate. This volume summarizes a conference hosted by the Institute of Medicine that illuminated these issues.