The objective of this prospective study was to identify risk factors for developing rhabdomyolysis in patients with doxylamine overdose. Patients who were admitted to a university teaching hospital between July 2000 and September 2005 due to doxylamine overdose were recruited. Demographic information, clinical variables, and laboratory data were investigated. Twenty-seven (M/F 12/15, age 33.2 +/-13.1 years) patients were enrolled. Sixteen (59%) of 27 patients developed rhabdomyolysis and three (19%) of 16 patients with rhabdomyolysis also developed acute renal failure. Patients who developed rhabdomyolysis differed from those who did not in the amount of doxylamine ingested, initial serum creatitnine and arterial pH. In multivariate regression analysis, the only reliable predictor of rhabdomyolysis was the amount of doxylamine ingested (P = 0.039). The amount of doxylamine ingested (>/= 20 mg/kg) predicted the development of rhabdomyolysis with a sensitivity of 81%, a specificity of 82%, a positive predictive value of 87%, and a negative predictive value of 75%.In conclusion, rhabdomyolysis following doxylamine overdose was common, occurring in 87% of patients who ingested more than 20 mg/kg. The amount of doxylamine ingested was the only reliable predictor for developing rhabdomyolysis following doxylamine overdose.