Objectives To investigate the changes in thalamic volumes in subjects at ultra-high risk (UHR) for psychosis. Subjects manifest changes which are similar to but different from those found in subjects with schizophrenia, and thalamic structural changes were often reported in schizophrenic subjects. Methods Thalamic volumes of 29 UHR subjects, 31 subjects with schizophrenia and 29 healthy controls, were measured from their T1-weighted coronal magnetic resonance (MR) images using manual tracing. Results UHR subjects showed greater right (6.243±0.848 cm3) and total (12.254±1.532 cm3) thalamic volumes compared to healthy control subjects (right thalamic volume=5.527±0.715 cm3, total thalamic volume=11.058±1.490 cm3) or patients with schizophrenia (right thalamic volume=5.855± 0.938 cm3, total thalamic volume=11.513±1.784 cm3). The difference was significant for right (F= 5.837, p=0.004) and total (F=4.217, p=0.018) thalamic volumes after intracranial volume had been accounted for as a covariate in ANCOVA. However, thalamic volume of subjects with schizophrenia showed no significant difference from controls. This difference was not affected by the presence of major depressive disorder or the magnitude of psychotic symptoms. Those among the UHR subjects taking antipsychotic agents did not show enlarged thalamic volume compared to controls. Conclusion Our findings suggest that the possibility of a volumetric alteration of the thalamus characteristic of the UHR state.