The purpose of this study was to investigate the thus far unstudied relationship between attachment style and level of substance dependency in a sample of substance dependent adults and non-clinical controls in Ireland. In a cross-sectional between-groups design, 40 clients (male=26; female=14; mean age=30) of Merchants Quay Drug Services Project and 40 undergraduates (male=18; female=22; mean age 33) participated. A purpose-designed questionnaire, the Relationship Questionnaire (RQ) (Bartholomew & Horowitz, 1991) and the Leeds Dependence Questionnaire (LDQ) (Raistrick et aI., 1994) were used. Predictor variables were attachment style, family structure, age of onset of alcohol/drug use and family history of substance dependence. The criterion variable was total LDQ score. Attachment style differed between service users and students (X2=24.786; df=3; p<0.001, two-tailed). Attachment style (F(1,67)=9.886; p<0.01), age of first alcohol use (F(1,67) 127.495; p<0.001), age of first drug use (F(1,67)=36.499; p<0.001) and interaction between family structure and family history of substance dependence (F(l ,67)=7.159; p<0.01) had significant main effects on LDQ levels. The results indicate that attachment theory provides a useful framework for understanding substance dependence, and generating clinically useful information with implications for the improvement of addiction prevention and treatment methods.