Disease registries are a special class of reporting laws. Since the objective is not to control a communicable disease, there is often no penalty for failing to report to a disease registry. Most disease registries are statewide and involve either cancer or occupational illness; some, such as the CDC registry of cases of toxic shock syndrome, are national. Reporting cases to the registry may be mandatory or voluntary, but it is always desirable to have a complete registry. These registries are used to determine the extent of certain problems in the community and to try to determine causes. If they are inaccurate, they may give false correlations and become useless for research and prevention.

Consider a cancer registry that contains only half the cases of a particular kind of cancer. If a local industry tries to determine whether its workers are exposed to something that causes the cancer, it will look at the rate among its exposed workers and the rate in the general population. If the company finds all the cases among its workers but the cancer registry has only half the cases in the general population, then an exposure that does not cause the cancer will look like it does by a factor of two. An epidemiologist may know that this is wrong, but the reporter from the local paper will not.