We report the results of a linkage study of eight markers on chromosome 19 in a sample of 24 families multiply affected with schizophrenia and related psychoses. This study forms part of a systematic search of the entire genome using microsatellite markers spaced at 10-20 cM intervals. These data provide no evidence for the presence of a gene of major effect on chromosome 19 under the assumption of genetic homogeneity or heterogeneity. We have attempted to describe the extent to which this region has been excluded and demonstrate that while it is possible to exclude near-dominant and recessive models under the assumption that all families are linked to the same locus, power for exclusion falls away rapidly when incomplete penetrance and heterogeneity are invoked.