Twenty-three adult patients (ages greater than 15 years) and 75 children with acute lymphoblastic leukemia were treated with similar intensive, sequential cytotoxic protocols (L-2). The adult patients have lower remission rate (78%) than the children (98%). The duration of remission and the length of survival are also shorter in adults. The incidence of central nervous system (CNS) relapse in adults (27.7%) is higher than in children (7.1%) suggesting that prolonged prophylactic intrathecal methotrexate as given to the children is more effective than the schedule used for adults where intrathecal methotrexate was given only in the first 2 months of therapy. The low incidence of CNS involvement in children on the L-2 protocol compares favorably with other series reported using a combination of cranial irradiation and intrathecal methotrexate. In both adults and children there seemed to be a higher incidence of CNS involvement in patients with initial white blood cell counts greater than 25,000 cells/mm3.