In previous papers we have shown that malnutrition is an adverse prognostic factor in the outcome of treatment in patients with standard-risk acute lymphoblastic leukemia. The reason why undernourished children do poorly as compared with well nourished children is that malnourishment leads to a diminished bone marrow reserve thus making necessary to delivery of suboptimal doses of maintenance chemotherapy (less than that calculated according to body surface). Undernourished children receive less 6-MP and MTX and relapse more frequently in the bone marrow, finally reflecting a 5 yr disease-free survival of 26% as compared with 83% in well-nourished children (p less than 0.001). In this paper we have analyzed the impact of the variable delivery of sub-optimal doses of maintenance chemotherapy in a group of 43 pediatric patient with standard-risk acute lymphoblastic leukemia. The 5 year disease free survival of children receiving suboptimal maintenance chemotherapy was 7% as compared with 65% for those receiving full doses of both 6-MP and MTX (p less than 0.001), and more frequent relapse involving the bone marrow in children receiving low doses (71% versus 3%, p = 0.000005). The impact of the variable suboptimal doses of chemotherapy was present in children with both normal and deficient nourishment status. It is concluded that the delivery of suboptimal doses of maintenance chemotherapy, due to bone marrow toxicity related to an abnormal myelopoietic reserve, is an adverse prognostic factor in the outcome to treatment of patients with lymphoblastic leukemia.