Abstract: The invention concerns a system for detecting chromosomal imbalance by in situ hybridization of fluorescent probes on interphase nuclei, comprising the following phases: hybridizing in situ fluorescent probes on two separate chromosomes; exposing each probe with a different fluorochrome; measuring the intensity signals corresponding respectively to each probe thus exposed, on an assembly of nuclei, said measurement being carried out within a control cell population and within a cell population subjected to detection; calculating a ratio between the signals corresponding to each of the probes, said ratio calculation being carried out on said control cell population to provide a reference ratio and said population subjected to detection; comparing the fluorescence ratio corresponding to the population subjected to detection with the reference ratio; and processing the result of said comparison to detect an inter-chromosomal imbalance.