The current communicable disease surveillance system in the United States largely relies on reporting of communicable diseases and conditions by both physicians and laboratories. Incomplete or inaccurate reporting of these diseases impairs the estimation of incidence rates from surveillance systems as well as hinders the implementation and evaluation of public health control measures. The extent of incomplete reporting has not been quantified for a large geographic area over time or for more than a few diseases. Therefore, the completeness of communicable disease reporting was studied using a retrospective cohort study at 8 large healthcare systems in North Carolina (NC) spanning a ten-year time period. The NC Department of Health and Human Services (NC DHHS) communicable disease surveillance system is based on “mandatory” reporting of more than 60 diseases and conditions and is a passive surveillance system. Diagnostic codes from healthcare system billing records were used to ascertain the eligible cases to be reported to the communicable disease surveillance system, and a unique identifier was used to match these eligible patients to the case-patients who were reported to the NC DHHS surveillance system. In addition, a validation study was also conducted to estimate positive predictive values of the diagnostic codes for communicable disease case ascertainment because these codes are widely used for both public health surveillance and research. Quantification of communicable disease reporting completeness is critical to understanding the impact on two public health surveillance system goals, that is, disease incidence rate estimation and public health initiation of disease transmission control measures. In addition, these analyses may guide the development of local, state and national strategies for improvement of disease reporting and surveillance.