We have recently shown that the levels of urinary 6-hydroxymethylpterin are highly elevated (3 to 20 fold) in a variety of human malignancies as compared to its urinary excretion in patients with nonmalignant diseases or normal healthy subjects. In the subsequent studies, this parameter has been shown to be a reliable index for accurately monitoring the response of patients with acute myeloblastic leukemia (AML) during chemotherapy. In this study, the excreted urinary levels of 6-hydroxymethylpterin as well as the bone marrow lymphoblast values were measured simultaneously in four patients with acute lymphoblastic leukemia (ALL) during antileukemic therapy. Various drug regimens employed for treatment have also been indicated. A good correlation was seen between urinary pterin levels and % blasts during treatment, thus accurately monitoring remission or relapse of the disease in response to the antileukemic therapy. These results again conclusively show that the simple noninvasive determination of 6-hydroxymethylpterin provides a reliable index of the total tumor load in acute lymphoblastic leukemic cases undergoing treatment.