Essential thrombocythemia (ET) is one of the diseases included among the myeloproliferative disorders in which trisomies for chromosomes 8 and 9 commonly occur. In ET, only a few patients are known to show clonal abnormalities. With fluorescence in situ hybridization (FISH), interphase cells can be evaluated and clones can be detected even though not revealed by conventional cytogenetic methods. By using FISH for enumeration of chromosomes 8 and 9 in bone marrow cells, we studied 22 patients with ET; 20 of them were investigated at the time of diagnosis when they were still untreated with myelosuppressive agents. Only two patients had trisomy 8; one of them was also found to have +8 with conventional cytogenetics. None of the patients had trisomy 9; two patients had borderline values in comparison to a control group. Thus, in ET, no increased frequency of patients with trisomy for 8 or 9 at the time of diagnosis could be detected with FISH.