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Rhabdomyolysis is a common and potentially lethal clinical syndrome that results from acute muscle fiber necrosis with leakage of muscle constituents into blood. Myoglobinuria is the most significant consequence, leading to acute renal failure (ARF) in 15%-33% of patients with rhabdomyolysis. Rhabdomyolysis occurs from inherited diseases, toxins, muscle compression or overexertion, or inflammatory processes, among other disorders. In some cases, no cause is found. We describe 475 patients from the Johns Hopkins Hospital inpatient records between January 1993 and December 2001 for the following discharge diagnosis codes: myoglobinuria, rhabdomyolysis, myopathy, toxic myopathy, malignant hyperthermia, neuroleptic malignant syndrome, and polymyositis. Of 1362 patients, 475 patients with an acute neuromuscular illness with serum creatine kinase (CK) more than 5 times the upper limit of normal (>975 IU/L) were included. Patients with recent myocardial infarction or stroke were excluded. The etiology was assigned by chart review. For all, the highest values of serum CK, serum creatinine and urine myoglobin, hemoglobin, and red blood cells were recorded. Forty-one patients had muscle biopsy within at least 2 months from the onset of rhabdomyolysis.Of the 475 patients, 151 were female and 324 were male (median age, 47 yr; range, 4-95 yr). Exogenous toxins were the most common cause of rhabdomyolysis, with illicit drugs, alcohol, and prescribed drugs responsible for 46%. Among the medical drugs, antipsychotics, statins, zidovudine, colchicine, selective serotonin reuptake inhibitors, and lithium were the most frequently involved. In 60% of all cases, multiple factors were present. In 11% of all cases, rhabdomyolysis was recurrent. Underlying myopathy or muscle metabolic defects were responsible for 10% of cases, in which there was a high percentage of recurrence, only 1 etiologic factor, and a low incidence of ARF. In 7%, no cause was found. ARF was present in 218 (46%) patients, and 16 died (3.4%). A linear correlation was found between CK and creatinine and between multiple factors and ARF, but there was no correlation between ARF and death or between multiple factors and death. Urine myoglobin detected by dipstick/ultrafiltration was positive in only 19%. Toxins are the most frequent cause of rhabdomyolysis, but in most cases more than 1 etiologic factor was present. Patients using illicit drugs or on prescribed polytherapy are at risk for rhabdomyolysis. The absence of urine myoglobin, by qualitative assay, does not exclude rhabdomyolysis. With appropriate care, death is rare.