Objective: The purpose of this study was to investigate the relationship between family dynamics and health locus of control in families who had a member with an ostomy. Design: The study used a cross-sectional design. Setting and subjects: Data were collected by means of questionnaires completed by 2 groups of families. Twenty-nine subjects in the ostomy group were registered with a local chapter of the United Ostomy Association. Thirty subjects in the non-ostomy control group were recruited from a local church, a university housing complex, and a hospital volunteer service. Instruments: The Family Dynamics Measure of 62 Likert-type items measured 6 bipolar dimensions of family dynamics. The Multidimensional Health Locus of Control Scale of 18 Likert-type items measured internal or external locus of control. Methods: Questionnaires were mailed to subjects by staff at the local ostomy association and other cooperating groups. Completed questionnaires were returned by mail to the researcher. Results: Subjects in the ostomy group perceived significantly greater control by powerful others and greater rigidity in the family compared with the non-ostomy group. When families were perceived as rigid, both groups reported significantly greater control by powerful others. Subjects in the ostomy group who reported greater role reciprocity in the family perceived greater health locus of control by powerful others. Conclusions: Health care professionals and family members (powerful others) play an important role in helping a person with an ostomy. Assessing family dynamics can help professionals identify which aspects of family life are associated with locus of control for the person with an ostomy.