The objective of this paper is to identify those factors, which are critical to the success of a maintenance operation in general and to apply them to a particular maintenance project. The project in question is the maintenance and evolution of a very large and complex banking application system for securities processing which has been in progress since several years. Eight factors are defined and evaluated in accordance with the existing literature on software maintenance and with the experience gained on several such maintenance projects. Each of the eight factors is scored according to a given metric scale. It is left to the evaluator to weigh and evaluate the significance of the individual factors. The study is based on empirical data collected over the duration of the project and is intended to contribute to the overall knowledge of software maintenance management.