Parameters of water metabolism were measured serially in nine patients with the syndrome of self-induced water intoxication and psychosis (SIWIP). Clinical and laboratory findings indicated that SIWIP patients are type A of the syndrome of inappropriate antidiuresis. Estimated 24-hour urinary excretion of creatinine and early morning urinary creatinine concentration measurements were used to calculate 24-hour urine volumes. Polyuria was considered present for male patients when excretion was estimated to be greater than 2,600 ml of urine/24 hours or early morning urinary specific gravity was less than or equal to 1.003. Male patients with a specific gravity of less than or equal to 1.003 predictably excreted 28,000 ml of urine/day. Severe hyposthenuria may be a biological marker for a population at risk to develop complications of SIWIP, including seizures, coma, and death.