Thirty-four children, including nine relapsed cases with acute lymphoblastic leukemia (ALL) having hyperdiploidy (greater than 50 chromosomes) were studied on clinical and cytogenetic characteristics. The majority of children initially with hyperdiploidy (greater than 50 chromosomes), who showed favorable prognostic features such as lower leukocyte counts, lower serum lactic dehydrogenase levels, ages between 2 and 10 years, or the presence of common ALL antigen, had the most favorable outcome among childhood ALL (5-year survival rate was 100%). Even nine children, who showed poor prognostic features such as ages over 10 years, leukocyte counts over 2 X 10(4)/mm3 or lymphomatous signs, had also the same favorable outcome. There were no differences in clinical features between 6 patients with additional chromosomal structural abnormalities and 19 patients without them. Duplication of the long arm of chromosome 1 was frequently observed as additional chromosomal structural abnormalities. Patients with hyperdiploidy (greater than 50 chromosomes) observed at relapse, who had the same favorable clinical features as those at diagnosis, had a poorer prognosis. These findings show that initial hyperdiploidy (greater than 50 chromosomes) is an independent favorable prognostic sign in childhood ALL and additional chromosomal structural abnormalities may not indicate a poor prognosis among childhood ALL with hyperdiploidy (greater than 50 chromosomes). On the other hand, relapsed children with hyperdiploidy (greater than 50 chromosomes) have not a favorable outcome after the onset of relapse.