Inefficient polyglutamylation is a mechanism of resistance to methotrexate (MTX) in childhood T-lineage acute lymphoblastic leukemia (T-ALL) and in acute myeloid leukemia (AML) in comparison with childhood c/preB-ALL. We analyzed the profile of MTX polyglutamylation in childhood c/preB-ALL, T-ALL, and AML (n = 45, 15, and 14, respectively), the activity of the MTX-polyglutamate synthesizing enzyme folylpolyglutamate synthetase (FPGS) (n = 39, 11, and 19, respectively) and of the MTX-polyglutamate breakdown enzyme folylpolyglutamate hydrolase (FPGH) (n = 98, 25, and 34, respectively). MTX-Glu_4-6 accumulation after 24 hours exposure to 1 μmol/L [³H]-MTX in vitro was lower in T-ALL (threefold) and AML (fourfold) compared with c/preB-ALL (P ≤ .001). The FPGS activity was twofold lower in T-ALL and AML than in c/preB-ALL samples (P < .01). FPGH activity was not different between c/preB-ALL and T-ALL, but threefold higher in AML (P < .001). FPGS, FPGH, and the ratio FPGS/FPGH were correlated with MTX-Glu_4-6 accumulation (r = .49, r = −.34 and r = .61, respectively). Multivariate analysis showed that FPGS, but not FPGH, was an independent contributor for MTX-Glu_1-6 accumulation, but not for MTX-Glu_4-6 accumulation. In conclusion, low FPGS activity is associated with low accumulation of MTX-Glu_4-6 in T-ALL and AML. For the group of AML as compared with the group of ALL, a high FPGH activity can play an additional role.