The Holyoake Codependency Index is being developed to measure the extent to which a person endorses codependent beliefs and attributions. A 28-item pilot version was administered to 39 male and 268 female clients of a family counseling agency. Factor analysis, used to identify the shortest version with acceptable reliability, yielded a 13-item final version comprising three subscales (external focus, self-sacrifice, and reactivity). The subscales correspond to key themes within the literature on codependency. Scores on each subscale correlated significantly in the predicted direction with relevant measures of psychological functioning; providing initial evidence of construct validity. The 13-item scale was administered to a general community sample of 303 women and the factor structure was fully replicated. Internal consistency of the subscales ranged from .74 to .84 with the family counseling sample and from .73 to .83 with the general community sample.