This paper presents a report of the results of a comparative epidemiological study of indicators of drug misuse in seven European cities. The study was carried out between 1982 and 1986 under the auspices of the Pompidou Group of the Council of Europe. The members of the expert group summarize the drug situation in the seven cities and critically examine a range of indicators that are sometimes assumed to reflect trends in the prevalence of drug misuse. They conclude that a valuable assessment of drug misuse problems can be provided by drawing together data from several indicators. It is essential that these agency-based data are complemented by surveys and other studies of drug misuse in the populations concerned. The members of the expert group suggest that a single organizational unit is needed to integrate and analyse data and epidemiological studies and discuss different models of how this can be achieved. Improvement in the consistency and quality of the data is essential if policies are to be based on a sound understanding of trends in drug misuse.