The volume of certain brain structures, particularly those in the anterior medial temporal lobe, may be reduced to a greater extent in hyponatremic/hypo-osmolemic polydipsic schizophrenics than other schizophrenic patients. To explore if volume reduction is specific to this particular brain region, and if it is fundamentally associated with polydipsia, we imaged the anterior hippocampi/subicula and adjacent temporal lobes in normal males (n = 10) and polydipsic schizophrenic patients with (n = 7) and without (n = 6) hypo-osmolemia. Bilateral hippocampal/subicular (p < .01), but not temporal lobe (p < .30), volumes were diminished in the hypo-osmolemics relative to the other two groups, who resembled each other on these measures. No recognized or putative factor could explain these findings. Thus anterior medial temporal lobe structures appear to be preferentially diminished in hypo-osmolemic polydipsic schizophrenics. Additional studies are needed to more precisely define these anatomic differences and their relationship to these patients' impaired water excretion and severe mental illness.