The establishment of the new diagnostic category, Schizotypal Personality Disorder (SPD), has stimulated biological studies of patients with this disorder. Such studies offer the potential of better understanding the diagnosis and treatment of SPD as well as more clearly defining the boundaries of the schizophrenic disorders. SPD has been studied in the clinical setting, in family studies of schizophrenia, and in the biological high-risk paradigm. In most cases, biological variables associated with schizophrenia have been evaluated. Decreased activities of plasma amine oxidase and platelet monoamine oxidase have been associated with SPD in the families of schizophrenics and in "biological high-risk" studies. Smooth pursuit eye movement (SPEM) impairment has also been associated with SPD in a "biological high-risk" study of college students. Inferior backward masking performance has been demonstrated in SPD patients in the clinical setting. Other studies using psychophysiological measures have been applied to subjects with psychological characteristics similar to DSM-III SPD and found biological abnormalities similar to those reported in schizophrenia. These studies are consistent with the possibility that some individuals with SPD may share common psychobiological abnormalities with schizophrenic individuals and may sharpen our understanding of SPD and its relationship to schizophrenia.