Typically, drug addicts show high incidence rates of offending and are responsible for a high proportion of offenses committed in large cities like Zurich. Moreover, a large proportion of drug addicts who are addicted to hard drugs, especially heroin, are concentrated in so-called needle parks of several major Swiss cities. In response, local and federal authorities in Switzerland decided to close the needle parks in 1992 and to approach drug addicts through different therapeutic offers. Among these measures was the selection of the most severely addicted persons for a program based on heroin prescription under strict medical control. Offending rates before and after admission to the program were evaluated, based on 319 subjects who started treatment on or after April 1, 1995, and who completed two follow-up interviews. Evaluation results showed that the health, social situation, and behavior of treated drug addicts substantially improved and that heroin prescription significantly reduced criminality. 13 references and 3 tables