While treatment with AZT has been found to be effective in prolonging survival and reducing the numbers of opportunistic infections in patients with AIDS, during the second year of administration of AZT an acceleration in mortality has been observed. The reasons for this are not known at this time. The study of what may be an AZT-resistant strain of HIV may benefit patients who have been and are still receiving AZT or another drug used in treating HIV ddC. It is hoped that the comparison of the effectiveness of AZT and ddC will benefit in the treatment of these patients.