An isochromosome for the long arm of chromosome 17,i(17q), is frequently found as an additional chromosome aberration to the Ph with advanced disease in the chronic myelocytic leukemia (CML). We studied an i(17q) in blood samples from two patients with CML in blast crisis with a biotinylated chromosome 17 specific alpha satellite deoxyribonucleic acid probe. G-banded karyotypes of these patients showed a dicentric i(17q), dic(17)(p11.2). Fluorescence in situ hybridization (FISH) delineated one normal chromosome 17 and one i(17q) among metaphase chromosomes; the latter showed a dicentric pattern. In most interphase nuclei of both patients, two fluorescence spots were observed. In some interphase nuclei, including mature neutrophils, the dicentric chromosome was discernible by its size and shape of the fluorescent spots. Three fluorescent spots were observed in a small proportion of interphase cells, and existence of a subclone with two normal chromosome 17 and an i(17q) was confirmed by examining a large number of metaphase plates. The results of FISH provided us with information of numerical and structural aberrations of chromosome 17 in interphase cells.