Fluorescence in situ hybridization (FISH) is currently developed to analyse chromosomal abnormalities of hemopoietic malignancies in several ways: description of chromosomal rearrangements using specific probes or chromosome painting; delineation of chromosomal breakpoints with probes previously localized to chromosomal bands; hybridization to interphase nuclei to detect numerical changes and, now, some structural abnormalities. Examples of usefulness of FISH to study hemopoietic malignancies are given.