Knowledge of the patterns of allelic loss has been useful in identifying tumor suppressor genes in many solid tumors. Although the loss of genetic material in acute lymphoblastic leukemias has been documented by cytogenetic studies and microsatellite typing, a global overview of losses of heterozygosity occurring throughout the genome was not yet available. We have performed a high resolution allelotype analysis in 63 childhood B-lineage acute lymphoblastic leukemia. A total of 247 microsatellite markers, evenly distributed along the autosomes were typed in blast and in remission samples from every patient. An average of 41 patients were informative for each marker. LOH at one or several loci was observed in 41 of the 63 patients (64%). The mean values for the fractional allelic loss (FAL) and the hemizygosity index, calculated for each patient, were 0.03 (range 0 to 0.23) and 0.024 (range 0 to 0.18), respectively. The most frequently involved chromosomal arms were 9p (36%), 12p (31%), 20q (15%), 6q (12%), 5p (10%) and 10p (10%). Three regions on chromosomal arms 9p, 12p and 6q were previously identified as the targets of recurring deletions, the target genes being identified for two of them (9p and 12p). The three new regions defined by this allelotype may contain tumor-suppressor genes implicated in the initiation or progression of childhood B-ALLs.