We present the nonrandom occurrence, frequency, and degree of immunophenotype association of the t(1;22)(p13;q13) in children with acute nonlymphocytic leukemia (ANLL). This karyotype anomaly occurred in leukemia cells from five of 445 (1.1%) children with newly diagnosed ANLL who were successfully studied by cytogenetic analysis at four European centers between January 1987 and January 1992. The occurrence of the t(1;22) was restricted to the French-American-British classification (FAB) subtype M7. The overall incidence in children with acute megakaryocytic leukemia (AMKL) was 27.8% (5/18 cases); in infants with AMKL, the frequency of the t(1;22) was 66.7% (4/6 cases). Three of the patients carrying this anomaly had a diploid karyotype, whereas in two cases a hyperdiploid karyotype was found. However, in all five patients, the t(1;22) was the only translocation event present at diagnosis. All patients received aggressive chemotherapy for acute myelogenous leukemia (AML). Two patients died within 15 months of diagnosis without entering remission. One of three patients who entered remission died 7 months after diagnosis, most likely from intramedullar hemorrhage. Only two of the five children with the t(1;22) who received autologous bone marrow transplantation (BMT) are alive and in complete remission (CR) 23 and 40 months after diagnosis, respectively. At the time of diagnosis, the age of the oldest child carrying the t(1;22) was 18 months. The cases with this chromosome anomaly were compared with an age-matched group of five children with AMKL lacking this translocation. The patients with the t(1;22) had a lower median value of the peripheral white blood cell (WBC) count and a higher median hemoglobin level than the patients from the matched group. In the latter cases, normocellular or hypercellular bone marrow (BM) was detected at diagnosis. In contrast, all children with the t(1;22) in our series had a hypocellular BM. Histological BM analyses were available in three of these patients and showed marked fibrosis. Other clinical and laboratory parameters showed no obvious differences between the matched groups. Despite intensive chemotherapy, AMKL in children appears to be associated with a poor prognosis. The clinical courses of the children with AMKL and the t(1;22) presented may be indicative of a beneficial effect of autologous BMT in this subset of patients.