The management of many chronic illnesses involves medications that must be injected on a frequent basis. With fewer support resources available, patients are increasingly being obliged to manage injectable medications themselves. Interferon beta-1a (IFNbeta-1a), recommended for the treatment of multiple sclerosis (MS), must be injected intramuscularly on a weekly basis. Patients are generally advised and taught to self-inject, if possible. This longitudinal study examined cognitive and affective contributions to the ability to self-inject and adherence to IFNbeta-1a over 6 months following initiation of medication. Participants were 101 patients with a relapsing form of MS. Injection self-efficacy expectations, injection anxiety, adherence expectations, method of injection administration, and 6-month adherence to IFNbeta-1a were fitted to a path analytic model. Pretreatment injection self-efficacy expectations were significantly related to 6-month adherence. This relation was mediated by the patient's ability to self-inject. Patients 'experienced level of injection anxiety was related to adherence but not to method of injection.