Chromosomal abnormalities that co-occur with psychiatric disorders can be useful direct pointers to the locus of susceptibility genes. Two families with pericentric inversions of chromosome 18, inv 18(p11.3 q21.1) and psychiatric illness have previously been described. We have fine mapped the chromosomal breakpoints of the rearrangement in a clinically well, inversion carrier from one of these families where other inversion carriers suffered from chronic schizophrenia or severe learning disability. Yeast artificial chromosomes (YACs) from the Whitehead/MIT physical maps of human chromosome 18 have been positioned relative to the chromosomal breakpoints and a number of YACs that span these breakpoints have been identified. Linkage and association studies have previously suggested these regions of chromosome 18q and 18p as candidate loci harbouring genes involved in bipolar disorder and schizophrenia.