The neurofibromatosis type 1 (NF1) gene has been considered to be a tumor suppressor gene, since the NF1 gene product downregulates the ras oncogene product (p21(ras)). In addition, children with NF1 show an increased incidence of myelogenous leukemia, including juvenile chronic myelogenous leukemia (JCML). We studied 8 Japanese JCML patients without NF1 for mutations in exons 21-36 of the NF1 gene by using polymerase chain reaction/single-strand conformation polymorphism analysis. This region was chosen because it includes two main hotspots in the NF1 gene as well as the functionally important domain, GTPase-activating protein-related domain, which mediates the downregulation of ras activity. One of the 8 JCML patients exhibited a G to T transversion at the third nucleotide of the codon GAG for Glu at amino acid residue 1699, which results in the heterozygous conversion to Asp (E1699D). This variation was detected neither in 65 healthy volunteers nor 50 NF1 patients. No other variations were detected in our JCML patients. We suggest that NF1 gene mutation does not occur frequently in JCML without NF1.