Thirty seven children with relapsed acute lymphoblastic leukemia (ALL), 25 B-lineage and 12 T-lineage, were analyzed for p53 alterations at different stages of the disease. Loss of heterozygosity (LOH) was detected in the relapse phase in three patients. p53 mutations were identified by single strand conformation polymorphism (SSCP) and sequencing analyzes in seven of the 37 ALL patients (19%); three B-lineage (12%) and four T-lineage (33%). Most of the mutations were identified in the relapse phase. In two exceptional cases, one of the mutations was indicated as a germ line and the other was already present at diagnosis. No p53 mutation was identified in any of the other 20 available bone marrow samples obtained at diagnosis. No correlation between the p53 status and clinical outcome could be determined. The majority of the mutations (four out of seven, 57%) were clustered at exon 5. Our data implicate that p53 exon 5 is a frequent site of mutations in relapsed childhood ALL.