A behavioral observation scale (Virginia Polydipsia Scale; VPS) for monitoring drinking patterns was developed and its reliability tested during 25 hours of tandem ratings among six patients with the syndrome of psychosis, intermittent hyponatremia, and polydipsia (PIPS). These ratings were compared to those collected from a control group of six psychiatric inpatients who were similarly observed for 25 hours. The scale was subsequently used to assess day-long drinking in a single PIPS patient. Results demonstrated that the VPS can be reliably administered by trained raters and that it clearly differentiates the drinking patterns of PIPS patients from controls. In addition, our findings highlight associations among drinking behaviors, psychiatric functioning and low serum sodium concentration. On balance, these results support using observational measures of drinking behaviors in future studies of PIPS patients.