Abstract. Objective: To determine the effects of lower-extremity positioning on cerebrospinal fluid opening pressure (CSFp). The authors believed that during lumbar puncture (LP), CSFp does not meaningfully decrease when the lower extremities are extended from flexion, as is often suggested. Methods: In a convenience sample of adult patients who clinically required LP in an urban emergency department, three sequential CSFp measurements were obtained in either sequence A (knee, hip, and neck flexion [90Њ], then extension, then flexion) or sequence B (extension, flexion, then extension) prior to CSF withdrawal. The neck was flexed at 30Њ when the lower extremities were flexed, while the thoracolumbar spine was kept in the neutral position for all measurements. Results: Nineteen patients were studied in each sequence. Although variable, overall withinpatient changes between positions were not clinically meaningful. Mean and 95% confidence intervals (95% CIs) for the decrease in CSFp from position 1 to position 3 (same position) were 0.2 cm H 2 O (1.7%) and 0.9 to Ϫ0.6 cm H 2 O (6% to Ϫ2.7%), respectively. Changing from flexion to extension decreased pressure measurements by a mean of 0.9 cm H 2 O (2.5%) [95% CI = 2.1 to Ϫ0.1 cm H 2 O (7.6% to Ϫ2.4%)].Changing from extension to flexion increased CSFp by a mean of 1.1 cm H 2 O (6.1%) [95% CI = 0.2 to 2.0 cm H 2 O (1.3% to 11.5%)], a statistically but not clinically meaningful change. Conclusions: Changing lower-extremity position did not meaningfully change mean CSFp. These data do not support the common suggestion that extending the lower extremities during LP meaningfully decreases CSF opening pressures. Key words: lumbar puncture; cerebrospinal fluid pressure; emergency medicine; procedures. AC-ADEMIC EMERGENCY MEDICINE 2001; 8:8-12 T RADITIONAL teaching implies that lowerextremity positioning (flexion vs extension) may alter the accuracy of opening pressure measurements obtained during lumbar puncture. Opening pressure measurements are sought in such disease states as meningitis, subarachnoid hemorrhage, and pseudotumor cerebri. It is suggested that if the opening pressure is elevated, the physician should extend the lower extremities for a more accurate reading.1-3 However, to the best of our knowledge, the effect of positioning on lumbar puncture pressure measurements has been subject to little study.Most previous studies on cerebrospinal fluid opening pressure (CSFp) have not used efficient or powerful experimental designs to address the effects of changing lower-extremity position. In some, the effect on CSFp of changing between supine and sitting positions has been measured. 4,5 In another, average CSF pressures with patients' lower extremities in one position were found to be higher than expected, 6 allowing indirect inferences regarding the effects of lower-extremity positioning on CSFp. Data obtained from this study were subject to a great degree of variability between different subjects and different procedures. This limits the statistical power and c...