The French National Identification System for Drugs and Other Substances (SINTES) is an original scheme gathering analytical information for synthetic drugs, both through police and customs' seizures in the entire country and collection of samples and questionnaires directly from the users by social field workers. Between July 1999 and June 2004, 9543 samples were included. Tablets (7004) were mainly containing MDMA (82%) and caffeine was the most frequent blended psychoactive substance. Mean MDMA dosage of tablets decreased from 1999 to 2003 and dosage for tablets bearing the same logo appeared to be highly variable. Notwithstanding the difficulties for data collection due to the illicit nature of these drugs, this surveillance and early warning system, which combines the cooperative efforts of law enforcement laboratories and social workers, provided relevant and timely information. It is accurate regarding the follow-up of trends in drugs' composition, and the identification of new or potentially dangerous substances, to the professionals, the public, and the European partners.