OBJECTIVES: Describe the clinical presentation, prevalence, and outcomes of concurrent serious bacterial infection (SBI) among infants with mastitis. METHODS: Within the Pediatric Emergency Medicine Collaborative Research Committee, 28 sites reviewed records of infants aged #90 days with mastitis who were seen in the emergency department between January 1, 2008, and December 31, 2017. Demographic, clinical, laboratory, treatment, and outcome data were summarized. RESULTS: Among 657 infants (median age 21 days), 641 (98%) were well appearing, 138 (21%) had history of fever at home or in the emergency department, and 63 (10%) had reported fussiness or poor feeding. Blood, urine, and cerebrospinal fluid cultures were collected in 581 (88%), 274 (42%), and 216 (33%) infants, respectively. Pathogens grew in 0.3% (95% confidence interval [CI] 0.04–1.2) of blood, 1.1% (95% CI 0.2–3.2) of urine, and 0.4% (95% CI 0.01–2.5) of cerebrospinal fluid cultures. Cultures from the site of infection were obtained in 335 (51%) infants, with 77% (95% CI 72–81) growing a pathogen, most commonly methicillin-resistant Staphylococcus aureus (54%), followed by methicillin-susceptible S aureus (29%), and unspecified S aureus (8%). A total of 591 (90%) infants were admitted to the hospital, with 22 (3.7%) admitted to an ICU. Overall, 10 (1.5% [95% CI 0.7–2.8]) had sepsis or shock, and 2 (0.3% [95% CI 0.04–1.1]) had severe cellulitis or necrotizing soft tissue infection. None received vasopressors or endotracheal intubation. There were no deaths. CONCLUSIONS: In this multicenter cohort, mild localized disease was typical of neonatal mastitis. SBI and adverse outcomes were rare. Evaluation for SBI is likely unnecessary in most afebrile, well-appearing infants with mastitis.