The aim of this study was to examine the effect of assertiveness and severity of drug addiction on the risk of relapse among drug addicts during a six months treatment. 90 Drug addicts completed the Research Interview on Addictive Behavior and the Rathus Assertiveness Schedule (RAS). The results showed that global assertiveness and Confrontation (a dimension of the RAS) are directly related to the risk of relapse, indicating that the higher the scores on these two variables the greater the risk of relapse. Other dimensions of the RAS (Defense of Rights and Personal Interests, Avoidance of Personal Confrontation, and Spontaneity) and the chronicity of drug use showed an inversed relationship to the risk of relapse, indicating that the higher the scores on those dimensions and on the chronicity of drug use the lower the risk of relapse. These results suggest caution when including assertion training for relapse prevention in drug addiction, and advise also caution when choosing the assessment instruments for assertiveness.