In this study, patients with Philadelphia (Ph) negative and positive chronic myelogenous leukemia (CML) were analyzed by unicolor- and dual color- (DC) fluorescence in situ hybridization (FISH) using abl and bcr cosmid probes. Unicolor- and DC-FISH analysis revealed a BCR-ABL fusion in a Ph-negative patient. DC-FISH to interphase nuclei revealed the BCR-ABL fusion, even though no metaphases were available for metaphase-FISH and conventional cytogenetic analysis. In the remaining cases, the findings of FISH were in agreement with the cytogenetic results.