Isochromosome of the short arm of chromosome 12 [i(12p)] is a highly specific chromosome abnormality of human testicular germ-cell tumors (TGCTs). It has been detected in 80% of cases. Other abnormalities that involved 12p-derived material have been observed in the remaining 20% of cases. In our study, cryptic rearrangements of the short arm of chromosome 12 were distinguished by fluorescence in situ hybridization (FISH) in eight cytogenetically abnormal TGCTs. This group included multiplicated material of 12p-arm in both i(12p)-positive and i(12p)-negative tumors. Such a common occurrence of chromosome 12 short arm rearrangements and overrepresentation of 12p-material confirms that yet unidentified gene(s) on 12p can play an important role in oncogenesis of TGCTs.