OBJECTIVE: Streptococcus anginosus group (SAG) bacteria have been associated with severe illness in patients with odontogenic neck abscesses. Magnetic resonance imaging (MRI) offers excellent diagnostic accuracy in neck abscesses. We sought to explore whether MRI findings could predict SAG-positivity. STUDY DESIGN: We included patients who had received emergency MRI for an acute odontogenic neck abscess and subsequent surgery with pus sampling. From the MR images, we measured abscesses and determined patterns of retropharyngeal, mediastinal, submandibular space, sublingual space, and visceral space edema. SAG-positive and -negative patients were compared regarding MRI findings along with demographic, clinical, laboratory, and surgical parameters. Data were analyzed using univariate models and a multivariate binary logistic regression model. RESULTS: Of 93 total patients, 46 patients (49%) were SAG-positive and 47 (51%) SAG-negative. The SAG-positive patients had larger maximal abscess diameters (36.2 vs 31.7 mm, P = .036) and a higher prevalence of mediastinal edema (ME) (43% vs 15%, P = .003). In the multivariate model, ME was the only statistically significant predictor of SAG positivity (OR = 7.455, P = .004). Regarding surgical outcomes, SAG-positive patients required transcervical incisions more often than SAG-negative patients (61% vs 36%, P = .004). CONCLUSIONS: ME detected on emergency MRI is highly suggestive of the involvement of SAG in odontogenic neck abscesses. These findings highlight the clinical utility of emergency MRI in patients with acute neck infections.