BACKGROUND: This study evaluated trends in adjuvant systemic treatment among breast cancer patients and analyzed the factors on which treatment choice was based. PATIENTS AND METHODS: Patients diagnosed with early stage breast cancer in 1990-2006 wre selected from the registry of the Comprehensive Cancer Centre South (n= 8261). The probability of receiving therapy was determined per characteristic for the periods 1990-1997, 1998-2001 and 2002-2006, separately. RESULTS: The use of any adjuvant systemic treatment increased from 37% in 1990-1997 to 51% in 1998-2001 and 53% in 2002-2006 (p for trend < 0.0001). In the period 1990-1997, lymph node status (positive vs. negative: probability ratio (PR) =25.8; 95% CI, 16.5-40.4) and age ( 60 vs. 35 years:PR=0.01; 95% CI, 0.00-0.02) were the main determinants of the likelihood of receiving chemotherapy. From 1998 onwards, age remained the most important factor in decreasing the likelihood of receiving chemotherapy. During 1990-1997 the use of hormonal therapy was mainly determined by positive lymph node status (PR=35; 95% CI, 25-49) and age ( 70 vs. 35 years: PR=9.3; 95% CI, 4.4-20), whereas positive hormone receptor status mainly affected hormonal therapy use (PR=17; 95% CI, 10-28) in the period 2002-2006. Marked differences were observed between hospitals in the adoption of adjuvant systemic treatment for node-negative patients. CONCLUSIONS: The impact of patient and tumour characteristics on treatment choice varied over time, reflecting major changes in the Dutch treatment guidelines. Patients older than 70 years received almost no chemotherapy.