Six hundred and fifty-six children with acute lymphoblastic leukemia (ALL) have been diagnosed in the five Nordic countries (Denmark, Finland, Iceland, Norway and Sweden) during the period from July 1981 through June 1985. Annual incidence of ALL was 3.6/100,000 children aged less than 15 years, with an incidence for males of 3.8 and for females of 3.4 respectively. Half of the children were younger than 5 years of age at diagnosis, with a peak incidence between 2-3 years of age. The leukemias were classified as Standard Risk (SR), Intermediate Risk (IR) or High Risk (HR) leukemia according to prognostic criteria at diagnosis. The remission rate was 95%. In children greater than or equal to 1 year of age with non-B-cell ALL at diagnosis, the Event-Free Survival (EFS) was 0.58; 0.65 for SR-children, 0.51 for IR-children and 0.52 for HR-children. WBC count at diagnosis was the most important prognostic factor and a WBC count of 11-20 X 10(9)/l was associated with the worst prognosis of all WBC values (EFS = 0.30), independent of other prognostic factors. Male sex was the second most important adverse prognostic criterion. The follow-up in January 1986 (observation time 6-54 months), showed that 442 of the 656 children (67%) were in complete continuous remission. The total results indicate a possibility to improve the prognosis for most of the risk groups of ALL with a more intensive treatment.