A person/years at risk (person/years) modified life table analysis system of computer programs, designed by NIOSH, is reviewed. The system is designed to analyze occupational cohort mortality data, although it can be modified to analyze other types of cohort data. System input data is described. Before analysis, the program edits the data for 14 criteria and either modifies it temporarily or deletes part or all of the worker's data. Parameters that may be specified for each analysis include: study begin data, study end data, races and sex to be included, and minimum duration of employment in exposed areas for cohort inclusion. Person/years are calculated in days. Parameters for person/year initiation are listed, and automatic calculation for the cohort is described. The system uses the World Health Organization International Classification of Disease code for the underlying cause of death. Means of taking revisions of the code into account are discussed. Calculations to compute expected deaths are examined. Verification of US mortality rates used by the system and of the distribution of person/years and expected deaths over age, time, duration of employment, and time since first employment is described. The output includes the person/years, observed deaths, and expected deaths displayed by the following: (A) any two of these variables: latency, duration of exposure, or dose, and (B) age by calendar period for each strata of the matrix of item 1. The authors conclude that these programs are advantageous because they include input data editing and modification, mortality rates for 89 causes of death, and simultaneous examination of observed and expected deaths by duration of employment (or dose), latency, age, and calendar time.