Genomic DNA from patients with idiopathic myelofibrosis (IMF) was screened by polymerase chain reaction (PCR) and conformation sensitive gel electrophoresis (CSGE) for mutations in the C-KIT gene (60 patients), as well as the C-FMS and FLT3 genes (40 patients). Intronic primers were used to amplify the entire coding region of both the C-KIT and C-FMS genes, and selected regions of the FLT3 gene. CSGE and direct DNA sequencing detected all previously reported as well as several novel polymorphisms in each of the genes. A novel c-fms exon 9 mutation (Gly413Ser) was detected in two patients. Its functional significance remains to be determined. The c-kit mutation Asp52Asn, previously described in two of six IMF patients in Japan, was not detected in this study. In addition, the reported c-fms mutations involving codons 301 and 969 were not identified. Therefore, in contrast to acute myeloid leukaemia, mutations in RTKs class III do not appear to play a significant pathogenetic role in idiopathic myelofibrosis.