DNA isolated from blood or bone-marrow samples from 18 patients with acute non-lymphocytic leukemia (ANLL) and 14 patients with acute lymphocytic leukemia (ALL) was analyzed for the presence of mutations in the N-ras gene. Using synthetic oligonucleotide probes we detected mutations in 5 cases of ANLL; 4 GGT----GAT transitions in codon 12 and one CAA----AAA transversion in codon 61. One case exhibited homozygosity for the mutation. No mutations could be detected at these codons in the DNA of the 14 ALL patients. In a follow-up study with 3 of the above 5 patients, the mutation could no longer be detected in 2 cases following successful induction of clinical remission by chemotherapy. However, the mutated N-ras persisted in one patient who did not achieve remission. We show that oligonucleotide hybridization is a sensitive assay for the detection of N-ras point mutations, which in ANLL could be used to follow the fate of the leukemic clone during (and after) therapy.