We set out to investigate the level of accordance of diagnosis and treatment of elderly breast cancer patients with national guidelines and to study predictors of deviation. Data on patient and tumor variables were collected from charts of 166 patients aged 70 years and older, diagnosed at our hospital in 2002-2004. Diagnostic work-up and treatment were compared with guidelines and reasons for deviation were recorded; 122 (74%) patients were diagnosed and treated in accordance with guidelines. Diagnosis was incomplete in 19 patients (11%). Surgery, radiotherapy, and hormonal therapy were withheld in 19 (11%), 11 (7%), and 9 (5%) patients, respectively. Guideline deviation was motivated in 18 patients (11%) (comorbidity n=11, patients' preferences n=5, age n=2), unmotivated in 18 (11%), and undeliberate in 8 (5%). Our study demonstrates that deviation from guidelines in elderly breast cancer patients mainly occurs due to a deliberate adjustment to patient's comorbidity and preference.