['To evaluate tamoxifen and aromatase inhibitor (AI) initiation over time and by patient characteristics among women diagnosed with breast cancer in a community setting.', 'We conducted a retrospective cohort study of 1,501 women age ≥ 18 years diagnosed with stages I to II invasive, hormone receptor-positive breast cancer from 2001 to 2008 in an integrated delivery system. Using automated pharmacy dispensings, we determined endocrine therapy receipt within 12 months of diagnosis. We used generalized linear models to estimate adjusted relative risks (RRs) with 95% CIs for any endocrine therapy use (v none), tamoxifen use (v none), AI use (v none), and AIs first (v tamoxifen). Each model adjusted for age, stage, body mass index, tumor size, lymph node status, comorbidities, other treatment, and diagnosis year.', 'Tamoxifen use was at its highest (56.9%) in 2001 and then decreased; AI use was lowest in 2001 (5.5%) and then peaked in 2005 (36.8%). In multivariate models, women age ≥ 65 years were less likely to use any endocrine therapy compared with women age 55 to 64 years (age 65 to 74 years: RR, 0.86; 95% CI, 0.78 to 0.96; age ≥ 75 years: RR, 0.71; 95% CI, 0.61 to 0.81). Women age ≥ 75 years were significantly less likely to begin AIs versus no treatment (RR, 0.46; 95% CI, 0.32 to 0.64) and versus tamoxifen (RR, 0.67; 95% CI, 0.46 to 0.97). Women with tumor sizes 1.0 to 1.9 cm and ≥ 2.0 cm were significantly more likely to use any endocrine therapy compared with women with tumor sizes < 1.0 cm (RR, 1.41; 95% CI, 1.23 to 1.61 and RR, 1.52; 95% CI, 1.27 to 2.81, respectively).'] Does endocrine therapy initiation from 2001 to 2008 vary by age at breast cancer diagnosis and tumor size?