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 MTXGlu(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.