From time to time in the paternity laboratory evidence for non-paternity is obtained in one or more genetic systems, while the remaining systems strongly suggest paternity. While this can occur by chance, such cases should be pursued further. Such a situation can be an important clue that silent alleles are present, if the evidence for non-paternity is limited to conditions of apparent opposite homozygousity (so-called “second order exclusions”). Rarely, such a situation can be a clue that the evidence for non-paternity is based on serologic irregularities; confirmation may be prudent. More commonly, especially when the evidence for non-paternity is incontrovertable, the situation provides a clue that a man related to the tested man is the father. We derive the theoretical framework for such an evaluation and review our experience with case material, which demonstrates that undisclosed unclehood is by no means rare.