Some clinicians working with families with alcohol or other drug problems continue to use the codependency model to guide their practice despite the limited empirical support for this approach. Research into codependency has been hampered by the lack of psychometrically sound instruments. The Holyoake Codependency Index (HCI; G. E. Dear & C. M. Roberts, 2000) is a 13-item self-report measure of codependent traits that has previously shown adequate to high reliability, initial evidence of construct validity, and an internal structure that is consistent across samples. In the 4 studies reported here, the internal structure of the HCI was confirmed using confirmatory factor analysis, and further evidence of construct validity was found in that the HCI subscales showed meaningful associations with other psychological and demographic variables.