Because a son's genotype at a Y-linked locus uniquely specifies his father's genotype at that locus irrespective of the maternal contribution, Y-linked polymorphisms show increased exclusion power over autosomal polymorphisms in paternity cases involving a male child. This advantage is even more obvious when the alleged father is deceased or otherwise unavailable for testing. In this situation, any individual connected by patrilineage to the alleged father may be tested in his place. The usefulness of the Y-linked tetranucleotide repeat locus Y-27H39 in deficiency cases was evaluated in a set of 41 families that had a deceased alleged father and that had been tested at GENE/MG with multilocal and unilocal DNA fingerprinting probes. In sixteen of these cases the proband (child tested) was male and there were male relatives. In the thirteen cases in which paternity was included, Y-27H39 would contribute significantly to the paternity index. In one of three cases in which there was exclusion by fingerprinting probes there was also exclusion by Y-27H39. Thus, Y-27H39 is useful in deficiency paternity cases and will be specially valuable in situations where autosomal polymorphisms have limited power, such as when there is only one male living relative of the deceased father. However, our experience is that Y-linked probes can only be applied in approximately 40% of cases.