ddC has been shown to have an antiviral effect, and AZT is known to significantly decrease mortality and to 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. This may be due to the emergence of AZT resistant virus isolated from some patients who have been on long-term AZT therapy. These isolates were still sensitive to ddC. A study of long-term effectiveness of ddC in patients with AIDS or advanced ARC who have been on long-term AZT therapy is warranted because (1) ddC has antiviral activity, (2) there is no blood toxicity associated with taking ddC, and (3) the effectiveness of ddC in test tube studies does not seem to be diminished by decreased effectiveness of AZT.