The complications of water intoxication are well documented in the medical literature. Less well appreciated is the frequent appearance of self-induced water intoxication in patients with schizophrenic disorders. Six such patients are described and compared with the findings in the literature. Nonedematous, nonhypovolemic hyponatremia is the invariable biochemical abnormality in this syndrome and its multiple causes are discussed, including the syndrome of inappropriate antidiuresis. Severe hyposthenuria (urinary specific gravity 1.003 or less) is the silent biological marker that always antedates the complications of self-induced water intoxication and schizophrenic disorders (SIWIS). While recognizing that all patients with polydipsia do not go on to develop water intoxication, this biological marker provides the means to detect patients who may be destined to develop SIWIS; early recognition may prevent the major complications of this syndrome.