Multiple measurements of serum osmolality (Sosm) and urine osmolality (Uosm) were assessed in 295 surgical patients in the past two years. These were performed with a recently developed osmometer which gives results at the bedside in one or two minutes. In six illustrative cases the value of these determinations is emphasized in trauma with free water retention, hypovolemic shock, respiratory insufficiency, sepsis, hyperalimentation with hyperosmolality, and renal failure. These osmolality measurements in the critically ill and injured patient have demonstrated that when the measured plasma osmolality is compared to the calculated osmolality and a significant discrepancy is found, there is accumulation of abnormal metabolite. Increase in these solutes carries ominous prognostic significance. Further, Uosm measurement is essential to the diagnosis and management of acute renal failure.