This work is a research in the field of systems science, emphasising the importance of applying models and theories that have been developed in this area. This work studies the possibility of using James Miller's living systems theory (LST) as a diagnostic tool. The application area was project management processes used when developing computerised information systems. The focus on the analyses was on the critical subsystems that process information. Based on this study it was found that LST function as a diagnostic tool according to the following criteria: it was possible to identify the critical subsystems in the application, the critical subsystems covered relevant information flows in the application and LST could make a unique contribution in the analyses of the application.