Chronic myelogenous leukaemia is a clonal myeloproliferative stem cell disease. Its cytogenetical hallmark is the Philadelphia chromosome (Ph) or the BCR/ABL fusion gene. Their identification is important both in the diagnosis and the follow-up of the disease. In our department we have investigated the BCR/ABL gene arrangement in 21 patients with fluorescence in situ hybridization. The aim of the analysis in freshly suspected patients without any previous therapy was to confirm diagnosis and mapping the ratio of Philadelphia positive cells. In contrast to the 95-100% Ph-positivity of mononuclear cells by classical cytogenetical examinations we found BCR/ABL gene arrangement only in various but always lower proportions. Therefore the latter examination gives a better representation of residual normal hemopoesis. Out of 9 patients who had received interferon treatment for at least 6 months, 4 gave a major, 4 a minor cytogenetical answer and in 1 case there was no cytogenetical response. Seven patients reached a complete and 2 a partial hematological remission. Among 5 other patients receiving interferon treatment, in 2 cases with double Ph-positivity we found a rapid progression. The data of 3 patients had to be excluded from the evaluation due to the so far short following time.