To identify a commonly deleted region of 13q14 in idiopathic myelofibrosis (IMF), we used fluorescence in situ hybridization analysis to test for deletion of the RB1 and BRCA2 genes, and the microsatellite loci D13S319 and D13S25, in a series of 25 patients. A further two patients with myelofibrosis secondary to polycythaemia vera and essential thrombocythaemia with reciprocal 13q translocations were studied in an attempt to further define the CDR. Twenty out of 21 patients with a cytogenetically normal chromosome 13 failed to show allelic loss with any of the four probes. In contrast, all four cases with cytogenetic deletion of 13q14 and both cases with 13q translocations involving 13q14 exhibited loss of RB1, D13S319 and D13S25. Loss of the BRCA2 locus was present in a single case only. Our results indicate that cryptic deletions of the 13q14 in myelofibrosis are rare. In addition, the genetic loss associated with cytogenetic 13q14 deletions or reciprocal translocations involving 13q14 is large and encompasses the gene-rich region around RB1, D13S319 and D13S25.