Purpose: Studies have reported disparities by age and race in the initiation of adjuvant trastuzumab for the initial treatment of older women with early-stage breast cancer, but less is known about its initiation in younger patients. Therefore, we assessed temporal trends and clinical and demographic factors associated with trastuzumab initiation in a large, population-based cohort of patients aged <64 years in 5 states. Methods: Using a cancer registry and claims-linked data set of 13,398 women with incident invasive breast cancer from 2006 to 2011, we identified 934 patients aged <64 years with HER2-positive stage I–III breast cancer. We assessed trastuzumab initiation within the first 9 months after diagnosis and conducted logistic regression analyses to assess sociodemographic and clinical factors associated with trastuzumab initiation. Results: From 2006 to 2011, trastuzumab initiation steadily increased in patients with node-positive (from 65% to 91%) and node-negative (from 39% to 75%) breast cancers. Several tumor-related factors were associated with trastuzumab initiation, including high histologic grades (adjusted odds ratio [aOR], 6.43; 95% CI, 3.27–12.65; and aOR, 3.25; 95% CI, 1.66–6.36, for grades 3 and 2, respectively), node-positive status (aOR, 1.88; 95% CI, 1.28–2.78; P=.001), tumor size >2 cm (aOR, 1.50; 95% CI, 1.04–2.16; P=.03), and hormone receptor–negative status (aOR, 1.51; 95% CI, 1.01–2.26; P=.04). We found a null effect of race. Conclusions: Adjuvant trastuzumab therapy for early-stage breast cancer has been widely disseminated among women aged <64 years. The initiation of this targeted therapy was associated with higher-risk features, consistent with practice guidelines.