The reliability of a system with a complex modular and redundant structure depends on many different factors, especially if failures within the system are to a certain extent to be diagnosed automatically. The influence of these factors is not clearly visible without detailed analysis. This paper describes a method of calculating the reliability of a system taking into account its structure and the reliability data of its modules, i. e. , failure rate, time necessary for automatic and for manual diagnonis, percentageof failures which are diagnosable automatically and mean repair time. A computer program based on this method enables limited resources to be used as efficiently as possible to increase the reliability of a system.