Adult patients with acute lymphoblastic leukemia (ALL) who are stratified into the standard-risk (SR) group due to the absence of adverse prognostic factors relapse in 40% to 55% of the cases. To identify complementary markers suitable for further treatment stratification in SR ALL, we evaluated the predictive value of minimal residual disease (MRD) and prospectively monitored MRD in 196 strictly defined SR ALL patients at up to 9 time points in the first year of treatment by quantitative polymerase chain reaction (PCR). Frequency of MRD positivity decreased from 88% during early induction to 13% at week 52. MRD was predictive for relapse at various follow-up time points. Combined MRD information from different time points allowed definition of 3 risk groups (P < .001): 10% of patients with a rapid MRD decline to lower than 10(-4) or below detection limit at day 11 and day 24 were classified as low risk and had a 3-year relapse rate (RR) of 0%. A subset of 23% with an MRD of 10(-4) or higher until week 16 formed the high-risk group, with a 3-year RR of 94% (95% confidence interval [CI] 83%-100%). The remaining patients whose RR was 47% (31%-63%) represented the intermediate-risk group. Thus, MRD quantification during treatment identified prognostic subgroups within the otherwise homogeneous SR ALL population who may benefit from individualized treatment.