This report presents the results and findings of a field trial conducted in 1999 on the treatment demand indicator. This field trial was conducted as part of a project which had the following aims: putting together a standard protocol for the TDI based on the results of former projects, recommendations and comments added during previous projects; conducting a field trial to collect treatment data following this protocol and the core item list from all EU member states and subsequent analysis of the collected data (comparability, validity, availability).