Mortality Schedules are probably one of the most overlooked resources available to genealogists. Yet, they are also one of the most interesting. Though they document less than 1/10th of the deaths which occurred,  they provide a historically significant snapshot during this time period. While today we consider heart disease and cancer as the major threats to modern life;  dysentery, small pox, cholera and consumption were the words that made life precarious at times for our ancestors. 

Mortality Schedules were to include the taking of names of all persons who died one year prior to 01 June 1850, 1860, 1870, 1880, 1885.  There are some exceptions and like the regular census, the mortality schedules are not complete. It is estimated that census takers missed at least 20 percent of the deaths they should have recorded. They were to record the state, county, the name of the person, age, sex, month died, birth place, marital status, cause of death and occupation. The enumerators were very inconsistant. Some records are very complete and some records leave much to be desired....but they are always worth checking when trying to locate an ancestor who can not be found on the regular census or when a death record can not be found. In many states where vital records were not kept it provides a nation-wide death register.

Remember that an index entry is only a reference to more detailed information found in census records themselves.  It is important that researchers consult the actual census records to which these indexes refer. 
Some of the mortality schedules are available on microfilm through the National Archives.  Most of the mortality schedules are available on microfilm through the Family History Library in Salt Lake City and its associated Family History Centers and thru most major libraries.   All statewide mortality schedules are indexed from 1850 to 1880.  For 1885 only South Dakota has been indexed.