BACKGROUND: Doxylamine is the most commonly intoxicated drug in the emergency room. This drug is relatively safe but is known to induce rhabdomyolysis and acute renal failure in rare occasions. We found the presence of microscopic hematuria in doxylamine intoxicated patients. But no previous studies have documented this hematuria. Our objectives of this study were to determine the incidence of microscopic hematuria after doxylamine overdose and to find the prognostic factors that contribute to this complication. METHODS: This study was conducted from 22 patients admitted to Kyung Hee Medical Center after doxylamine intoxication during the period from January 2001 to December 2003. Using the protocol made beforehand, the amount ingested, past history and laboratory results were recorded. Rhabdomyolysis was defined as serum myoglobin over 300 ng/mL or serum creatine phosphokinase (CK) over 1, 000 IU/L. Data were analyzed using SPSS program with t- test, Fisher's exact test and discriminant analysis. RESULTS: The microscopic hematuria was detected in 63.6% of patients. The amount ingested per body weight, presence of rhabdomyolysis and the time when the muscle enzymes reach highest level were related to the hematuria. CONCLUSION: The incidence of microscopic hematuria was higher when more than 30 mg per body weight of doxylamine was ingested than less this amount. Microscopic hematuria suggests the presence of kidney and urinary tract injury. Urine pH of hematuria is over 7.5. Our findings provide no support for the belief that the ferrihemate injures the kidney of doxylamine ingested patients.