Importance: At many institutions, computed tomography with iodinated intravenous contrast medium is the preferred imaging modality for staging of the neck in squamous cell carcinoma of the head and neck. However, few studies have specifically assessed the diagnostic accuracy of computed tomography for determining the presence or absence of extracapsular spread (ECS)., Objective: To determine the accuracy of modern, contrast-enhanced, multidetector computed tomography in the diagnosis of ECS of cervical lymph node metastases from squamous cell carcinoma of the head and neck., Design, Setting, and Participants: Retrospective observational study at an academic tertiary referral center among 100 consecutive patients between May 1, 2007, and February 1, 2012, who underwent a lateral cervical neck dissection for squamous cell carcinoma of the head and neck with neck metastases of at least 1 cm in diameter on pathologic assessment. Exclusion criteria included malignant neoplasms other than squamous cell carcinoma, a delay in surgery longer than 6 weeks from the time of staging computed tomography, and prior treatment of the neck or recurrent disease or a second primary., Main Outcomes and Measures: Each patient was independently assigned a subjective score for the presence of ECS by 2 Certificate of Added Qualification-certified neuroradiologists according to a 5-point scale. Receiver operating characteristic curves were generated, and sensitivity, specificity, accuracy, positive predictive value, and negative predictive value were calculated for each observer., Results: The areas under the receiver operating characteristic curve for observers 1 and 2 are 0.678 (95% CI, 0.578-0.768) and 0.621 (95% CI, 0.518-0.716), respectively. For observer 1, the positive and negative predictive values for the detection of ECS were 84% (95% CI, 68%-93%) and 49% (95% CI, 36%-62%), respectively. For observer 2, the positive and negative predictive values for the detection of ECS were 71% (95% CI, 57%-82%) and 48% (95% CI, 32%-64%), respectively., Conclusions and Relevance: Computed tomography cannot be used to reliably determine the presence of pathologic ECS. Radiologic findings suggestive of ECS should not be relied on for treatment planning in squamous cell carcinoma of the head and neck.