© 2017, Society of Surgical Oncology. Purpose: Pediatric breast malignancies are rare, and descriptions in the literature are limited. The purpose of our study was to compare pediatric and adult breast malignancy. Methods: We performed a retrospective cohort study using the National Cancer Data Base comparing patients ≤21 years to those >21 years at diagnosis (1998–2012). Generalized linear models estimated differences in demographic, tumor, and treatment characteristics. Cox regression was used to compare overall survival. Results: Of 1,999,181 cases of invasive breast malignancies, 477 (0.02%) occurred in patients ≤21 years. Ninety-nine percent of adult patients had invasive carcinoma compared with 64.8% of pediatric patients with the remaining patients having sarcoma, malignant phyllodes, or malignancy not otherwise specified (p < 0.001). Pediatric patients were twice as likely to have an undifferentiated malignancy [relative risk (RR) 2.19; 95% confidence interval (CI) 1.72–3.79]. Half of adults presented with Stage I disease compared with only 22.7% of pediatric patients (p < 0.001). Pediatric patients were 40% more likely to have positive axillary nodes (RR 1.42; 95% CI 1.10–1.84). Among patients with invasive carcinoma, pediatric patients were more than four times as likely to receive a bilateral than a unilateral mastectomy compared with adults (RR 4.56; 95% CI 3.19–6.53). There was no difference in overall survival between children and adults. Conclusions: Pediatric breast malignancies are more advanced at presentation, and there is variability in treatment practices. Adult and pediatric patients with invasive carcinoma have similar overall survival.