Objective: Ventriculoperitoneal (VP) shunting is a common procedure for treatment of idiopathic normal pressure hydrocephalus (NPH). As part of the workup for NPH, an external lumbar drain is commonly performed, with patients undergoing pre- and post-lumbar drain assessment of their symptoms. Neuropsychological assessment is often used to assist in the determination of candidates who might benefit from VP shunting. The present study was conducted to assess the cognitive effects of external lumbar drain (ELD) in a sample of patients with a clinical diagnosis of NPH who later received a VP shunt. Method: 44 participants were administered the Mini Mental State Exam (MMSE), Trail Making Test, Letter Fluency, Category Fluency, Hopkins Verbal Learning Test – Revised (HVLT-R), and Digit Span Forward and Backward. Assessments were conducted pre- and 72–96 hours post-ELD to determine the percent change in test-retest scores. Results: Subjects showed an improvement of >10% on both verbal fluency measures and a decrease of >10% on HVLT-R delay following ELD. Age effects were seen on Trails A, Category Fluency, and HVLT-R delay, with younger participants showing greater improvements than older subjects on Category fluency and HVLT-R delay. Conclusion(s): Measures of verbal fluency and delayed recall showed greatest sensitivity to ELD in participants with a clinical diagnosis of NPH. These results may have implications for the selection of NPH patients who might benefit from receiving a VP shunt. However, more research is needed to determine optimal cognitive test procedures for selecting appropriate candidates for VP shunting.