The incidence and consequences of fluid overload in the surgical ICU (SICU) have not been well defined, but may influence length of stay, days requiring mechanical ventilation, and mortality. Forty-eight consecutive patients admitted to our SICU were prospectively monitored for acute changes in weight and its impact on clinical management and outcome. When defined as a gain greater than 10% from their preoperative or premorbid weight (or an approximately 20% increase in total body water), 40% of patients had fluid overload. Patients were divided into three groups: those who had gained less than or equal to 10%, those with a weight gain between 11% and 20%, and those with greater than 20% increase in weight. Significant differences were found with respect to vasopressor dependence, colloid administration, and mortality. When indexed by initial Acute Physiology and Chronic Health Evaluation (APACHE II) mortality prediction scores, all groups had similar degrees of illness. On average, presumably due to volume limitations, patients were inadequately nourished during 85% of their SICU stay. Our results suggest that the morbidity of fluid overload can be significant, and warrants a fresh look at the methods of intraoperative fluid resuscitation.