BACKGROUND: Because doxylamine succinate (DS) is an over-the-counter medicine, it can be obtained easily and is frequently used in suicidal attempts. Patients usually recover without serious complications, but occasionally rhabdomyolysis and even death can occur in DS intoxication. In this study, the authors tried to find out the independent predictors of high peak serum CK levels, i.e. probable rhabdomyolysis in DS intoxication. METHODS: The medical records of 41 patients who visited a hospital for DS intoxication from January 1, 2002 to April 30, 2003, were reviewed retrospectively. RESULTS: In the group of DS only, initial occult blood of urine (P=0.003), initial WBC count (P=0.003) and confusion (P=0.007) were the best predictors of the peak serum CK level (2=0.724). In the group of DS with other drugs intoxication, initial creatinine level (P=0.003) and initial occult blood of urine (P=0.007) were the best predictors of the peak serum CK level (r2=0.784). In the cases of rhabdomyolysis patients, the time taken for the CK level to be increased over 1,000 IU/L was 1.9level to be increased over 1,000 IU/L was 1.9+/-0.6 days. CONCLUSION: In DS only intoxication, occult blood in initial urine analysis, initial high WBC count and confusion can be thought of as useful clinical predictors for high peak serum CK level case. In DS with other drugs intoxication, initial creatinine level and initial occult blood of urine can be considered as the best predictors. More than 2 days will be needed for the observation of serious complications in DS intoxication.