Some two hundred psychiatrists from all corners of the globe were gathered together in April for a week in Copenhagen, under the auspices of the World Health Organisation and the Alcohol, Drug Abuse and Mental Health Administration of the National Institutes of Health of the United States Government (ADAMHA). This meeting was the culmination of a programme of several years preparatory work by representatives of these two organisations, which had its origins in an initiative during Dr Gerald Klerman's period as administrator of ADAMHA. The stated objectives of both the preparatory work and the conference itself were to review the current state of work in classification and diagnosis of mental disorders, to identify gaps in present knowledge, and to define priorities for future collaborative research. Implicit in this programme of work is a realisation that there are many fundamental aspects of diagnosis in psychiatry that can only be studied fruitfully by international collaborative studies. Acceptance of this principle is, in itself, a sign of the progress that has taken place in this field in the last 15 years or so.