A data processing apparatus and method is disclosed. The data processing apparatus comprises a processor unit operable to execute data processing instructions, a processor state register within the processor unit operable to store processor state information associated with a data processing instruction being executed by the processor unit and a processor state storage circuit operable to store the processor state information provided by the processor state register for transmission to a diagnostic system. The processor state storage circuit is operable, in response to a processor state request signal from the diagnostic system requesting the stored processor state information, to prevent transmission of the stored processor state information to the diagnostic system when the processor unit is in a predetermined operating state. Hence, when the processor unit is in an operating state which may generate irrelevant or misleading processor state information, this processor state information is prevented from being provided to the diagnostic system. It will be appreciated that preventing such information from being sent to the diagnostic system ensures that only relevant information is analysed by the diagnostic system, and that any statistical data generated by the diagnostic system is accurate.