AIDS is a syndrome that represents the most severe form of infection with the retrovirus HIV. Opportunistic infections, uncommon malignant neoplasms, and intractable immunologic deficiency are hallmarks of AIDS. To date, the syndrome in the U.S. is seen mainly in epidemiologically restricted populations. It appears to be transmitted in a manner analogous to hepatitis B. There is no available means of reversing the immune deficit. Therapy is restricted to treating the complicating infections and tumors. Those having been diagnosed with P. carinii pneumonia may benefit from therapy with AZT. The spread of HIV and the syndrome can only be reduced by preventive measures until an effective and safe vaccine can be developed.