BACKGROUND: We previously reported reduced volume of the caudate nucleus in patients with SPD, and this time we used a new method of magnetic resonance imaging (MRI) to measure the shape of the caudate head. We believe that MRI shape analysis complements the conventional MRI volume measurements. METHODS: MRI scans were used to measure the shape of the caudate nucleus in 15 right-handed males with SPD who had not been previously treated with antipsychotics; an additional 14 matched normal control individuals were measured. The head of the caudate nucleus was measured using the shape index (SI), which measures the extent to which a shape deviates from the shape of a sphere, according to MRI processing tools. RESULTS: Individuals with SPD who had never used antipsychotics had significantly higher (sharper) head shape index scores in the caudate nucleus, with a right-handed bias, which correlated with control individuals. In addition, for individuals with SPD, higher right and left caudate head SI scores were significantly associated with poorer neuropsychological executive functioning in visuospatial memory and auditory/verbal working memory, respectively. CONCLUSION: These data demonstrate the value of measuring the volume and morphology of important brain regions, supporting a correlation between intrinsic caudate nucleus pathology and cognitive abnormalities in schizotypal disorders unrelated to antipsychotics.