ddC has been shown to demonstrate an antiviral effect. AZT has been shown to significantly decrease mortality and reduce the frequency of opportunistic infections in patients with AIDS or advanced ARC. After 1 year of AZT therapy, the effectiveness tends to diminish and patients progress with more opportunistic infections and higher mortality rates. Because of the demonstrated antiviral activity, absence of hematologic toxicity, and lack of cross tolerance in laboratory studies of ddC, a study to investigate the long-term effectiveness of ddC in patients with HIV infection who have received AZT therapy is warranted.