A family is reported in which the monozygotic co-twin of a schizophrenic proband was diagnosed bipolar 1 and their mother had a history of unipolar major depression. Although their clinical manifestations varied, the ill members of this family shared an abnormality in P300 not found in the asymptomatic siblings. In 14 families, linkage to the 5q11-13 region was excluded when affection status was defined solely by P300 latency independently of the clinical findings. Linkage was also excluded when the analysis was restricted to the families that had no cases of bipolar illness and when the schizophrenic phenotype was narrowly or broadly defined. It is concluded that biological markers such as P300 and eye tracking may help to clarify the overlap of different types of psychosis and help to define the phenotype for linkage analyses.