The Drugs Information and Monitoring System (DIMS) in The Netherlands is a toxicoepidemiologic monitor of drug markets that was established in 1992. Its main focuses are to identify the compounds of synthetic drugs, describe prevalence and trends, and identify health risks. Here we discuss the insights gained in the Ecstasy market, based on the weekly testing of more than 100 drug samples, and key information of synthetic drug users delivering drug samples and personnel participating in the DIMS network. Pills used as Ecstasy may contain a wide variety of compounds. The percentage of samples containing MDMA increased slowly reaching almost 75% in 1996, but decreasing sharply in 1997. Amphetamines ("speed" and "ice") and experimental varieties were found in at least one third of the pills. Origins and effects of this development are discussed, as well as the risk assessment. In 1998 the percentage of MDMA pills increased more than ever before, indicating among other things that consumers prefer the conventional product. However, the use of "speed" and other drugs may also be stimulated by the decrease in 1997 of the percentage of MDMA pills. With more new types of drugs likely in the next century, a monitor such as DIMS provides important surveillance and data for public health and preventive aims.