Purpose: The clinical significance of methotrexate (MTX)-induced hepatic toxicity in children with acute lymphoblastic leukemia (ALL) is poorly defined. Therefore, we conducted a study to determine whether intensive MTX therapy could be safely delivered despite isolated serum ALT elevations in children with ALL. Patients and Methods: A total of 243 children with B- precursor ALL were treated with extended pulses of oral divided-dose MTX (dMTX). Serum ALT levels were measured approximately every 7 weeks during therapy, as well as after its cessation. By protocol design, treatment was continued without modification in the presence of ALT elevations if there was no other evidence of liver dysfunction. Results: Of 239 assessable patients, 159 (66.5%)had an ALT level ≤ 180 IU/L during therapy and 28 patients (17.6%) had one or more values ≤ 720 IU/L. After the completion of therapy, only 17 of 104 assessable patients have had one or more elevated ALT value. Eight of these 17 patients (47%) are hepatitis C virus (HCV)-seropositive. The remaining nine children had subsequent normal or near normal ALT values, and none have clinical evidence of liver disease. Conclusion: Our data show that MTX can be safely delivered without dose modification in patients with isolated ALT elevations and that continued therapy does not lead to clinically apparent liver disease. ALT elevations are not a reliable predictor of the presence or extent of hepatic injury, and persistently increased ALT values following the completion of ALL therapy are rare in the absence of HCV infection. Continued MTX therapy allows for increased dose- intensity and may improve outcome in children with ALL.