Placement of a protective cervical collar is common in cases of acute head trauma. However, the effect of this collar on intracranial pressure is uncertain. This prospective study examined the change in measured cerebrospinal fluid pressure (CSFP) after the application of a rigid Philadelphia collar in 20 adult patients undergoing lumbar puncture. CSFP averaged 176.8 mm H2O initially and increased to an average of 201.5 mm H2O after collar placement (range 0 to 120). Although this difference of 24.8 mm H2O is statistically significant (P = .001), it is uncertain if this would be clinically important. Nonetheless, this small increment in pressure could be significant in patients who already have an elevated intracranial pressure.