Dynamically generating expanded user messages, in a computer system having a logging tool for creating a log file. The log file comprises log messages produced in response to events occurring during execution of an application program. Firstly, the application program source code is pre-analysed to extract a plurality of program components, including comments. The different component types and comments associated with them are ordered into tabular form. An individual log message is read from the log file and its program components are determined. Comments associated with these program components are read from the relationship tables. The comments are used to supplement the original log message by combining them to produce expanded user messages in a user-friendly format. These expanded user messages may be further grammatically parsed into natural language, in order to provide the end user with more meaningful diagnostic information.