The fusion of techniques of classical cytogenetics and molecular biology has led to the establishment of the fluorescence in situ hybridization (FISH) technique. This method, allowing detailed chromosomal investigations in metaphase spreads as well as in interphase nuclei, has enabled pathologists to detect molecular changes in precancerous lesions such as carcinoma in situ of the testis. In particular, its application to paraffin material has allowed the correlation of chromosomal aberrations with morphological and immunohistochemical characteristics. Thus, recurrent chromosomal aberrations found in invasive testicular tumours, such as the occurrence of an isochromosome i(12p), are already present in preinvasive stages, indicating an early event in carcinogenesis. Recent work, published in this issue, presents the diagnostic impact of these investigations.