At the first chromosome 18 workshop held at the 1997 World Congress on Psychiatric Genetics (WCPG) in Santa Fe, NM, several studies were presented that suggested the presence of a bipolar disorder (BP) as well as a schizophrenia (SZ) susceptibility locus on chromosome 18. Although the fact that several independent studies all pointed to a susceptibility locus (or loci) on chromosome 18, the observation that these studies identified nonoverlapping candidate regions was disappointing at least from the viewpoint of molecular genetics aiming at cloning the respective gene(s). Together, the data suggested four possible regions of considerable size that contained a susceptibility gene. At the 1998 WCPG chromosome 18 workshop in Bonn, Germany, less data were submitted and with the exception of a few studies, most data were nonsupportive or negative. Although some refinements were made to the previous candidate loci, overall the picture has not changed in that we are still confronted with the same four potential loci on chromosome 18 for BP and/or SZ, i.e., 18p11.2 and 18q12.1-q12.3 for BP and SZ, and 18q21-q22 and 18q23-qter for BP. So far, no other psychiatric phenotypes show evidence for a susceptibility locus on chromosome 18. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88:263-270, 1999.