The National Institutes of Health (NIH) Clinical Center in Bethesda, Maryland, is the nation's largest hospital devoted entirely to clinical research.

At the NIH Clinical Center, clinical research participants—more than 480,000 since the hospital opened in 1953—are active partners in medical discovery, a partnership that has resulted in a long list of medical milestones, including development of chemotherapy for cancer; the first use of an immunotoxin to treat a malignancy (hairy cell leukemia); identification of the genes that cause kidney cancer, leading to the development of six new, targeted treatments for advanced kidney cancer; the demonstration that lithium helps depression; the first gene therapy; the first treatment of AIDS (with AZT); and the development of tests to detect AIDS/HIV and hepatitis viruses in blood, which led to a safer blood supply. Patients come from all 50 states and from around the world.

Currently, there are about 1500 clinical research studies in progress at the NIH Clinical Center. About half are studies of the natural history of disease, especially rare diseases, which often are not studied anywhere else. What researchers learn by studying rare diseases often adds to the basic understanding of common diseases. Most other studies are clinical trials, which often are the first tests of new drugs and therapies in people. The clinical trials at the NIH Clinical Center are predominantly Phase I and Phase II, often first-in-human to test safety and efficacy.