IMPORTANCE: Liposuction is one of the most common cosmetic surgery procedures in the United States. Tumescent liposuction, in which crystalloid fluids, lidocaine, and epinephrine are infused subcutaneously before cannula-assisted aspiration of fat, can be performed without intravenous or general anesthesia, often at outpatient facilities. However, some of these facilities are not subject to state or federal regulation and may not adhere to appropriate infection control practices. OBJECTIVE: To describe an outbreak of severe group A Streptococcus (GAS) infections among persons undergoing tumescent liposuction at 2 outpatient cosmetic surgery facilities not subject to state or federal regulation. DESIGN: Outbreak investigation (including cohort analysis of at-risk patients), interviews using a standardized questionnaire, medical record review, facility assessment, and laboratory analysis of GAS isolates. SETTING AND PARTICIPANTS: Patients undergoing liposuction at 2 outpatient facilities, one in Maryland and the other in Pennsylvania, between July 1 and September 14, 2012. MAIN OUTCOMES AND MEASURES: Confirmed invasive GAS infections (isolation of GAS from a normally sterile site or wound of a patient with necrotizing fasciitis or streptococcal toxic shock syndrome), suspected GAS infections (inflamed surgical site and either purulent discharge or fever and chills in a patient with no alternative diagnosis), postsurgical symptoms and patient-reported experiences related to his or her procedure, and emm types, T-antigen types, and antimicrobial susceptibility of GAS isolates. RESULTS: We identified 4 confirmed cases and 9 suspected cases, including 1 death (overall attack rate, 20% [13 of 66]). One instance of likely secondary GAS transmission to a household member occurred. All confirmed case patients had necrotizing fasciitis and had undergone surgical debridement. Procedures linked to illness were performed by a single surgical team that traveled between the 2 locations; 2 team members (1 of whom reported recent cellulitis) were colonized with a GAS strain that was indistinguishable by laboratory analysis of the isolates from the case patients. Facility assessments and patient reports indicated substandard infection control, including errors in equipment sterilization and infection prevention training. CONCLUSIONS AND RELEVANCE: This outbreak of severe GAS infections was likely caused by transmission from colonized health care workers to patients during liposuction procedures. Additional oversight of outpatient cosmetic surgery facilities is needed to assure that they maintain appropriate infection control practices and other patient protections.