We report a 22-year-old male patient with untreated seizure disorder, presenting with increased frequency of seizures followed by encephalopathy. Laboratory evaluation showed severe hypernatremia (175 meq/l sodium), rhabdomyolysis, and acute renal failure (ARF). Excessive insensible water loss in hot and humid weather, associated with an inability to obtain adequate water replacement, led to a hyperosmolar state (plasma osmolality, 398 mOsm/kg). He was vigorously treated with hypotonic fluid supplement and, further, needed dialysis therapy (peritoneal dialysis followed by hemodialysis) for acute renal insufficiency. The patient survived without any neurological sequelae, but the clinical course was complicated by acute deep vein thrombosis. This case represents what we believe is a unique report in the literature of severe hypernatremia developing via the pathogenic mechanism outlined above and the complication of acute peripheral venous thrombosis, which has not been reported in adults. The purpose of this report is to emphasize hyperosmolarity as a newly described cause of rhabdomyolysis, ARF, and a hypercoagulable state.