Personality traits are major determinants of social behavior influencing various diseases including addiction. Twin and family studies suggest personality and addiction to be under genetic influence. Identification of DNA susceptibility variants relies on valid and reliable phenotyping approaches. We present results of psychometric testing of the Icelandic NEO-FFI in a population sample (N= 657) and a sample recruited for a study on addiction genetics (N= 3804). The Icelandic NEO-FFI demonstrated internal consistency and temporal stability. Factor analyses supported the five-factor structure. Icelandic norms were compared to American norms and language translations selected for geographical and cultural proximity to Iceland. Multiple discriminant function analysis using NEO-FFI trait scores and gender as independent variables predicted membership in recruitment groups for 47.3% of addiction study cases (N= 3804), with accurate predictions made for 69.5% of individuals with treated addiction and 43.3% of their first-degree relatives. Correlations between NEO-FFI scores and the discriminant function suggested a combination of high neuroticism, low conscientiousness and low agreeableness predicted membership in the Treated group.