From June 1971 to May 1974, 41 children, aged four months to eleven years, with acute lymphoid leukaemia were treated by a schedule which included prophylactic cranial irradiation and intrathecal methotrexate injection after remission (phase II) and combination cytostatic treatment for 2 1/2 years (phase III). Induction of remission (phase I) was usually achieved with vincristine and a corticosteroid. Complete remission, now lasting 18 to 54 months, was obtained in 18 patients (44%), in nine of them without treatment for up to 24 months. Twelve patients (29%) had bone-marrow recurrence, five (12%) CNS and three (7%) testicular recurrence. One child died of liver failure in complete remission. Another nine patients, who had been pretreated according to a different schedule and were in their first remission for six to twelve months, were added to the programme from phase II onwards. Three of them now are in complete remission for 60 to 88 months, seven to 30 months after treatment had been stopped. The results indicate a significantly improved prognosis of acute lymphoid leukaemia in children.