Folylpolyglutamate synthetase (FPGS) is responsible for the metabolism of natural folates and a broad range of folate antagonists to polyglutamate derivatives. Recent studies indicated increased accumulation of methotrexate (MTX) polyglutamates (MTX-PG) in blast cells as a predictor of favorable treatment outcome in childhood acute lymphoblastic leukemia (ALL). We determined the expression of FPGS activity in blasts from children with ALL at diagnosis and after treatment with MTX as a single agent, before conventional remission induction therapy. The levels of enzyme activity in ALL blasts at diagnosis (median of 689 pmol/h/mg protein) were significantly higher (P = .003) than those found in acute nonlymphoblastic leukemia (ANLL) blasts (median of 181 pmol/h/mg protein). Comparable lineage differences in normal lymphoid versus nonlymphoid cells suggest a lineage-specific control of FPGS expression, FPGS activity increased in ALL blasts after in vivo exposure to MTX. The median increase in FPGS activity was significantly higher (P = .003) in B-lineage ALL (188%) than in T-lineage ALL (37%). Likewise, the percentage of intracellular long chain MTX-PG (Glu3–6) was significantly higher (P = .02) in B- lineage ALL (92%) than in T-lineage ALL (65%), consistent with higher FPGS activity in B-lineage blasts. This finding could explain, at least in part, the superior outcome in children with B-lineage ALL treated with antimetabolite therapy.