Surveillance of communicable diseases in the United States depends on the reporting of cases by primary physicians. It is widely recognized, however, that significant numbers of such cases are not reported. Reporting rates for many communicable diseases have never been determined. In this study, discharge records of 11 hospitals in Washington, DC were searched for cases of selected communicable diseases, and the percentage of these cases reported was determined. Five hundred and seventy of 93,563 (0.61%) patients hospitalized over the study period had one of these communicable diseases. Reporting rates for each were as follows: viral hepatitis, 11%; H. influenzae meningitis, 32%; salmonellosis, 42%; meningococcal meningitis, 50%; shigellosis, 62%; tuberculosis, 63%; total cases, 35%. There is indirect evidence that low reporting rates are not restricted to the area studied. Supplemental reporting by medical laboratories, hospital infection control, and record room personnel were suggested as additional soruces of case reports.