This study was carried out between March and December 1985, on 50 drug addicts, including 45 heroin users, hospitalized for detoxication. We compared the clinical informations obtained at the time of admission with the toxicological informations obtained from blood and urinary samples. If the drugs said to have been used over the last 48 hours are always found, in 52% of these cases some other drugs are found too. The reasons of this difference between clinical and toxicological informations are analysed as well as their consequences on diagnosis and treatment. This data finally shows the importance of systematic toxicological analysis in the treatment of drug addicts as well as in the epidemiological studies.