The computer program MISCAN is developed for use in evaluation of mass screening for disease. The program uses Monte Carlo simulation. It produces output on the results of screening projects and on the effects of screening on morbidity and mortality on the individual and population level. The calculations are based on models of the natural history of the disease and of the impact of screening on the natural history. The approach is such that considerable flexibility exists in specifying the structure of the model and its parameters. The program consists of two parts. The DISEASE part can be used for simulating the epidemiology of the disease when no screening is taking place; it requires input on the population and on the disease process. The SCREENING part is to be used in combination with the DISEASE part. It is intended for simulation of the results and effects of a screening project. It requires input on the properties of the screening tests, the consequences of early detection by screening, and the policy (ages and intervals between screens) of the project. MISCAN can be used for finding model assumptions regarding the disease process and the impact of screening that give a good explanation of the observed results of a screening project. Such an analysis proceeds in two steps. First, MISCAN is used to calculate simulated results of the project, based on specific assumptions. Next, these results are tested against the observed results, in order to assess the acceptability of the assumptions. MISCAN can also be used for optimization of the screening policy by simulating the cost and benefit components of a large number of different screening policies.