State and local public health officials rely on health-care providers, laboratories, and other public health personnel to report the occurrence of notifiable diseases to state and local health departments. Without such data, trends cannot be accurately monitored, unusual occurrences of diseases might not be detected, and the effectiveness of intervention activities cannot be easily evaluated. In the United States, requirements for reporting diseases are mandated by state laws or regulations, and the list of reportable diseases in each state differs. In October 1990, in collaboration with the Council of State and Territorial Epidemiologists, CDC published Case Definitions for Public Health Surveillance (MMWR 1990;39[No. RR-13]), which, for the first time, provided uniform criteria for reporting cases. This report provides updated uniform criteria for state health department personnel to use when reporting the nationally notifiable infectious diseases listed in Part 1 of this report. A revision date is listed for each case definition that has been revised. Newly generated case definitions that have not been published previously are designated as "adopted" on the specified date. Case definitions for some infectious conditions not designated as nationally notifiable are included in Part 2 of this report. Some of these conditions may have been nationally notifiable or may become so; definitions are included here to facilitate interpretation of data for these diseases. These conditions may be reportable in some states.