Four groups of respondents--non-users, users of psychoactive substances, users of legal substances and users of illegal substances--categorized on the basis of a survey of a representative sample of 3,690 secondary school students on the island of Mallorca, were correlated with 13 psycho-social variables, the results of which showed distinct characteristics for each of the four categories of respondents. Between two poles consisting of non-users and users of illegal substances, there is a category of users of legal substances, while all three categories form a continuum from one end to the other. This continuum supports the view that drug abuse is a sequential learning process that leads from simple drug use to drug addiction. The authors believe that it is essential to identify characteristics of specific groups among secondary school students in order to implement a realistic and effective drug abuse prevention programme in a given school. A prevention programme should meet the needs of the target group; this study identifies such needs and shows how they can be assessed. Many drug abuse prevention programmes in schools have been ineffective because they have failed to respond to the realistic requirement of specific school groups.