Essential thrombocytosis (ET) is a clonal, chronic myeloproliferative disorder (MPD) originating from a multipotent stem cell. To date no specific cytogenetic marker has been found in ET. It was recently reported that chromosomal aberrations have been detected by fluorescence in situ hybridization (FISH) in patients with normal karyotypes or nonanalyzable metaphases. Therefore, we evaluated whether trisomies 8 and 9, which are commonly found in MPDs, can be detected in ET by FISH and compared the results with chromosome analysis. Peripheral blood mononuclear cells of patients with essential thrombocytosis were studied by classical chromosome banding and by FISH. We used biotin labeled α satellite of chromosome 8 and biotin labeled β satellite of chromosome 9 as probes for the FISH studies. FISH detected 5 patients with trisomy 8 and 5 with trisomy 9 of the 18 patients evaluated. No trisomy was found by cytogenetic studies. The trisomies were detected by FISH in only a minority of the cells. No correlation was found between the presence of a trisomy and clinical characteristics. FISH is a sensitive method for the detection of trisomes 8 and 9 in patients with ET. The common finding of these chromosomal aberrations in MPD suggests that genes associated with myeloid proliferation are located on these chromosomes. Standardization of interphase cytogenetics is needed before this technique can be accepted for routine use in the clinic.