A group of 43 pediatric patients with standard-risk ALL were studied. Thirty-seven per cent of them presented with malnutrition at diagnosis. Malnourished children had a significantly worse outcome than well-nourished children. Five-year DFS was 83% for well-nourished children (WNC) and 26% for under-nourished children (UNC) (p less than 0.001). Relapses presented more frequently in the bone marrow in UNC than in WNC (56% vs 7%, p less than 0.0001). The doses of maintenance chemotherapy had to be reduced in 68% of UNC and 11% of WNC (p less than 0.005); the doses of maintenance myelosuppressive chemotherapy (6-MP, oral MTX and hydroxyldaunorubicin) received by UNC were approximately 50% of those received by WNC (p less than 0.01). The correlation between malnutrition and compromised treatment was 0.92. Malnutrition might be included as an adverse prognostic factor in acute lymphoblastic leukaemia (ALL).