The progression of immunodeficiency due to HIV infection can be delayed by using AZT. The benefits of AZT in adults with AIDS and severe AIDS-related complex (ARC) appear to last for approximately 12 to 18 months, at which time most patients have progressive deterioration. Recently published literature has described a reduced sensitivity of HIV isolated from patients after prolonged AZT treatment. Although the clinical significance of this is unclear, it makes the development of new antiretroviral drugs important.