Although recent studies have suggested that cessation of imatinib (IM) in chronic myeloid leukemia patients can be associated with sustained response, further validation is needed to explore predictive factors. In a prospective, multicenter study, chronic phase patients were eligible for cessation of IM therapy after more than 3 years if they had no detectable BCR-ABL1 transcript for at least 2 years. A total of 48 patients with a median age of 47 years (19-74 years) were enrolled. Twenty patients received IM for post-transplant relapse. After a median follow-up of 15.8 months (1.4-28.2 months) after IM discontinuation, nine of the non-transplant group lost undetectable molecular residual disease (UMRD) and major molecular response (MMR), whereas none of the 20 patients in the transplant group experienced UMRD loss. Probabilities for sustained MMR and UMRD were 64.4% and 66.3% in the non-transplant group, respectively. Of nine patients re-treated with IM, eight patients re-achieved MMR at a median of 1.7 months (0.9-2.8 months). Seven of these patients re-achieved UMRD at a median of 5.6 months (2.8-12.1 months). Previous transplantation, IM duration, and UMRD duration were significantly associated with sustained molecular responses. Our data strongly suggest that immunological control contributes to sustained suppression of residual leukemia cell expansion and that IM can be safely discontinued in patients with post-transplant relapse.