Repeated PCR analysis was performed on bone marrow and/or peripheral blood samples from 4 CML patients in complete cytogenetic remission during treatment with IFN-alpha. Two patients became PCR-negative. One was negative for the analyses carried out from the 9th to the 30th months, but reverted to PCR positivity 10 months after IFN was reduced from 1.5 x 10(6) IU/day to 1 x 10(6) IU and given on alternate days. Although the dose was again raised to 3 x 10(6) IU/day, 8 months later her peripheral blood cells were still PCR-positive, but remained persistently Ph'-negative. Another patient became PCR-negative at the 42nd month and remained so at the last analysis performed 3 months later. Two patients were persistently PCR-positive. Cytogenetic relapse was documented in both, in one while still on full therapy. Ph'-positive metaphases reappeared in the other patient 7 months after discontinuing IFN-alpha therapy.