['To evaluate the discriminatory ability of maternal serum creatine kinase (SCK) as a test for ectopic pregnancy (EP).', 'Serum creatine kinase concentrations were obtained prospectively from symptomatic patients being evaluated for early abnormal pregnancy. Serum creatine kinase concentrations from all patients and from a subset of these patients with maternal serum beta-hCG concentrations < 6,500 mIU/mL (conversion factor to SI unit, 1.00) were analyzed with descriptive statistics, receiver operator characteristic (ROC) curve analysis, and calculations of predictive values.', 'A university hospital emergency room.', 'Fifty-six patients with intrauterine gestations (25 with beta-hCG concentrations < 6,500 mIU/mL) and 23 patients with EP (20 with beta-hCG concentrations < 6,500 mIU/mL) were studied.', 'For all patients and the subgroup with beta-hCG concentrations < 6,500 mIU/mL, mean SCK levels were not significantly different between ectopic and intrauterine gestations. For all patients and the subgroup with beta-hCG concentrations < 6,500 mIU/mL, the areas under the ROC curves did not demonstrate discriminatory ability of the SCK test. The highest positive predictive value of an elevated SCK for EP was 52% using the SCK concentration of 70 U/L, and this was seen in the subgroup of patients with beta-hCG values < 6,500 mIU/mL.'] Does serum creatine kinase predict ectopic pregnancy?