A prevertebral soft tissue measurement exceeding 4 to 5 mm at C3 on a lateral spine radiograph is considered to be evidence of cervical spine injury. The objective of this study was to determine the sensitivity of the prevertebral soft tissue measurement at C3 in patients with proven cervical spine fractures or dislocations and to determine if this measurement correlates with the location or mechanism of injury. Consecutive patients 16 years of age or older who were admitted from July 1988 to June 1995 to a tertiary referral hospital with a discharge diagnosis of cervical spine fracture or dislocation were retrospectively studied. Patients were excluded if an interpretable lateral cervical radiograph taken within 24 hours of the injury was unavailable, medical records were unavailable or incomplete, the injury was caused by penetrating trauma or attempted hanging, or retropharyngeal air was present on the lateral radiograph. For each study patient, the earliest available lateral radiograph was obtained, and the prevertebral soft tissue measurement at the inferior aspect of C3 was recorded. All medical records and reports of imaging studies were reviewed. Two hundred thirty-two patients were identified and 21 were excluded, leaving 212 study patients. Injuries were classified as high (C1 to C2), low (C3 to C7), anterior, or posterior. For each patient the mechanism of injury was inferred from the fracture pattern according to established criteria. For all patients the sensitivity of a prevertebral soft tissue measurement at C3 of > 4 mm was 66% (95% confidence interval [CI] 59, 72). For C1 to C2 (n = 71) and C3 to C7 (n = 138) injuries, the sensitivities were 64% (95% CI 56, 78) and 64% (95% CI 56, 72), respectively. For anterior (n = 95) and posterior (n = 70) injuries the sensitivities were 64% (95% CI 54, 74) and 64% (95% CI 52, 75), respectively. There was no statistically significant difference in the prevertebral soft tissue measurement at C3 for high versus low injury, anterior versus posterior injury, or mechanism of injury. These results show that the prevertebral soft tissue measurement at C3 is an insensitive marker of cervical spine fracture or dislocation and does not correlate with the location or mechanism of injury.