Although Post-traumatic Stress Disorder (PTSD) is common among patients seeking care at medical clinics, little is known about the performance of screening instruments for this disorder in these settings. Previous studies of acute trauma populations using the PTSD Checklist (PCL) have suggested that scores of 45-50 provide the best discrimination between cases and noncases. We gave the PCL to 1,225 randomly selected women enrolled in an HMO. After interviewing a sample of 261 of these women using a structured, clinician-administered PTSD interview, we compared the results of the PCL to the clinician interviews over a range of possible cut scores using Receiver Operating Characteristic analysis. The optimum balance of sensitivity and specificity for this population was a score of 30, yielding a sensitivity of.82 and specificity of.76. The positive and negative likelihood ratios for this cut score were 3.40 and 0.24, respectively. By comparison, the use of 45 as a cut score would result in very low sensitivity (.36) in this setting. The lower cut score found in this study may indicate that the use of previously published cut scores of 45-50 may not optimize the function of the PCL as a screening tool outside of acute trauma settings due to an unacceptably high number of false negative cases.