This study aimed to investigate the prevalence of and factors associated with non-adherence to medication amongst a sample of breast cancer patients. 131 women with stable disease were interviewed and completed standardised psychological measures. 55% of women reported non-adherence to medication frequently or occasionally, with younger women and those who disliked taking their medication being significantly less adherent (P = 0.015, P = 0.001). Women who deliberately omitted taking their tablets occasionally or frequently had significantly lower scores, indicative of a weaker influence, on 'internal' and 'powerful others' dimensions of health locus of control (P = 0.032, P = 0.009). Despite a life-threatening diagnosis, patients may not adhere to medication representing a potential missed opportunity for health gain and waste of resources. Furthermore, interpretation of clinical trial data may be misleading without adherence information. More research is needed to identify those at risk for non-adherence. If other routes of administration are available these options should be discussed with patients to maximise efficacy of therapy.