Nonadherence to adjuvant endocrine breast cancer treatment adversely affects disease-free and overall survival. Clinical predictors of nonadherence may allow for specific interventions to reduce nonadherence and improve survival. The aim was to investigate whether clinical characteristics predict nonadherence. Clinical characteristics and information on adherence were obtained from 417 patients with breast cancer in a population-based prospective cohort from southern Sweden using patient charts, pathology reports, and questionnaires filled out at the 1- and 2-year follow-up visits. At the 1- and 2-year follow-up visits, 36 (8.6%) and 33 (9.7%) patients were nonadherent, respectively. Thirteen of the nonadherent patients declined treatment and were never prescribed endocrine treatment. A body mass index (BMI) < 25 kg/m(2), preoperative current smoking, and drinking alcohol less often than twice a month predicted nonadherence at the 1-year [relative risk (RR), 5.24; 95% confidence interval (CI), 2.75-9.97] and the 2-year visits (RR, 4.07; 95% CI, 2.11-7.84) in patients with at least two of these clinical characteristics. When low histologic grade (I) was added to the model, having at least two of these four clinical characteristics predicted nonadherence at the 1-year (RR, 4.94; 95% CI, 2.46-10.00) and the 2-year visits (RR, 4.74; 95% CI, 2.28-9.87), the two profiles had a sensitivity ranging from 60.6% to 72.7%, whereas the specificity ranged from 68.0% to 78.4%. Nonadherence at the 1-year visit was associated with an increased risk for early breast cancer events (HR, 2.97; 95% CI, 1.08-8.15), adjusted for age and tumor characteristics. In conclusion, two clinical profiles predicted early nonadherence and may allow for targeted interventions to increase adherence if validated in an independent cohort.