Billing records from the outpatient clinics of a uni versity medical center were used to identify report able communicable diseases. Patient charts were re viewed to check the accuracy of all cases of commu nicable diseases not reported to the local health department. Thirty-three percent of the cases iden tified as one of 20 communicable diseases, using the ICD-9-CM system, were found to be incorrectly coded. This study documents a lack of specificity (nu merous false positives) when using encounter form data and ICD-9-CM codes to identify communicable diseases in an outpatient setting.