This review of paracentric inversions in man includes what we know of the behaviour and reproductive consequences of paracentric inversions from other species. Observations of naturally occurring inversions in several species of plants and animals and results of experiments with mutagenically induced inversions in the mouse are discussed. From a review of 184 cases, it is concluded that most of the paracentric inversions in man are harmless and that the risk of heterozygotes having a child with an unbalanced karyotype is low. However, in some cases, it is difficult, if not impossible, to distinguish between a paracentric inversion and a paracentric insertion, the risk in the latter case being about 15%. Caution is also necessary in interpreting the results of prenatal diagnosis for heterozygotes of paracentric inversions, because of the possibility of a variety of unpredictable unbalanced chromosome products.