This invitation for co-operation is addressed to all interested groups. Its purpose is data collection for clinical verification of symptoms from homeopathic proving and collection of clinical symptoms not derived from homeopathic pathogenetic trials (provings). To date no consensus exists on this topic. The European Committee for Homoeopathy (ECH) research sub-committee has created a working group on this topic. This paper is the result of a review of papers on the topic, exchanges between members and a meeting.