Methotrexate (MTX) is one of the most widely used drugs for the treatment of childhood acute lymphoblastic leukemia (ALL) and is commonly given in high doses. However, the rationale for high-dose MTX (HDMTX) has been challenged recently. To determine whether higher MTX polyglutamate (MTXPG) concentrations in ALL blasts translate into greater antileukemic effects, 150 children with newly diagnosed ALL were randomized to initial treatment with either HDMTX (1,000 mg/m2 intravenously over 24 h) or lower-dose MTX (30 mg/m2 by mouth every 6 h x 6). ALL blasts accumulated higher concentrations of MTXPG and long-chain MTXPG (MTXPGLC) after HDMTX (P < 0.00001). Of 101 patients evaluable for peripheral blast cytoreduction, MTXPG concentrations were higher in patients whose blast count decreased within 24 h (P = 0.005) and in those who had no detectable circulating blasts within 4 days (P = 0.004). The extent of inhibition of de novo purine synthesis in ALL blasts was significantly related to the blast concentration of MTXPGLC (IC95% = 483 pmol/10(9) blasts). The percentage of patients with 44-h MTXPGLC exceeding the IC95% was greater after HDMTX (81%) than LDMTX (46%, P < 0.0001). These data indicate that higher blast concentrations of MTXPG are associated with greater antileukemic effects, establishing a strong rationale for HD-MTX in the treatment of childhood ALL.