Monitoring minimal residual disease (MRD) in patients with acute lymphoblastic leukemia (ALL) is a useful way for assessing treatment response and relapse. We studied the value of MRD and showed a correlation with relapse for 34 adult patients with ALL. MRD was evaluated by real-time quantitative polymerase chain reaction (RQ-PCR) with probes derived from fusion chimeric genes (BCR/ABL) (n = 12) or PCR-based detection of clonal immunoglobulin and T cell receptor gene rearrangements (n = 16), or both (n = 6). We analyzed 27 of the 34 patients who could be examined for MRD on day 100 after induction therapy. The overall survival (OS) rate (45.0%) and relapse-free survival (RFS) rate (40.0%) at 2 years in complete remission (CR) patients with MRD level ≥ 10⁻³ (n = 12) were significantly lower than those in CR patients with MRD level <10(-3) (n = 15) (OS rate 79.0%, RFS rate 79.4%) (log-rank test, P = 0.017 and 0.0007). We also applied multicolor flow cytometry for comparison with MRD results analyzed by PCR methods. The comparison of results obtained in 27 follow-up samples showed consistency in 17 samples (63.0%) (P = 0.057). MRD analysis on day 100 is important for treatment decision in adult ALL.