Between 1974 and 1982, 43 children less than 2 years of age were treated in the hematology department of Hospital Saint-Louis for acute lymphoblastic leukemia (ALL). Of the patients who presented before 18 months of age, 80% had a WBC > 100,000 μl and/or a great tumor bulk. As a result of our experience, treatment regimens have been changed here from conventional chemotherapy to a very intensive program with a heavy induction (vincristine, daunorubicin, cyclophosphamide, prednisone, and L-asparaginase) and monthly reinductions with the same drugs plus ArA-C, without maintenance. Prophylaxis included CNS irradiation (16–24 Gy) after 12 months of age, plus intrathecal methotrexate. Complete remission (CR) occurred in 78% before 18 months and in 100% between 18 and 24 months of age at diagnosis. In this report the probability of a prolonged CR (33% at 2 years) was the same before and after 12 months of age. However, younger patients were more intensively treated. The prognosis for children less than 1 year of age who received very intensive chemotherapy has greatly improved, with a significantly higher probability of long CR (p < 0.02). Presently, 10 of 43 children are in CR 27 months to 8 years after diagnosis. Of 18 patients aged less than 1 year at diagnosis, four are in CR. No relapse occurred after 23 months. None of these patients presented with important sequellae, with the exception of one child who suffered from severe bacterial meningitis. An aggressive chemotherapy program is indicated in patients less than 2 years of age. The feasibility of this mode of treatment in young patients is possible only with the help of specific supportive care.