AZT is effective in reducing mortality in patients with AIDS who receive the drug after the first episode of Pneumocystis carinii pneumonia (PCP) and in patients with advanced ARC. However, AZT therapy has been associated with significant toxicities. In addition, the effectiveness of AZT appears to decrease during the second and third years of therapy. For these reasons, the development of alternative therapy that would be at least as effective but less toxic is of great importance. The drug ddI is an antiviral agent that inhibits replication (reproduction) of HIV with less apparent toxicity than AZT. The major dose-limiting toxicities found in the Phase I studies have been pains in the feet and legs of 2 patients initially receiving 12 mg/kg/day and 12 patients receiving daily doses of 25.8 to 51.2 mg/kg; symptoms began 8 to 27 weeks after initiating ddI treatment. These neuropathy-like symptoms have generally not been associated with significant abnormalities in nerve conduction studies and patients have reported marked improvement in symptoms within 1 to 2 weeks of discontinuing ddI. Some patients have resumed ddI treatment at a reduced dose after resolution of their symptoms. Studies indicate that ddI remains active in the body for at least 12 hours. This indicates that benefits of ddI might be achieved with a low frequency of drug administration.