BACKGROUND. Children treated with cerebrospinal fluid (CSF) shunts to manage hydrocephalus frequently develop shunt failure and/or infections, conditions that present with overlapping symptoms. The potential life-threatening nature of shunt infections requires rapid diagnosis; however, traditional microbiology is time consuming, expensive, and potentially unreliable. We set out to identify a biomarker that would identify shunt infection. METHODS. CSF was assayed for the soluble membrane attack complex (sMAC) by ELISA in patients with suspected shunt failure or infection. CSF was obtained at the time of initial surgical intervention. Statistical analysis was performed to assess the diagnostic potential of sMAC in pyogenic-infected versus noninfected patients. RESULTS. Children with pyogenic shunt infection had significantly increased sMAC levels compared with noninfected patients (3,211 ± 1,111 ng/ml vs. 26 ± 3.8 ng/ml, P = 0.0001). In infected patients undergoing serial CSF draws, sMAC levels were prognostic for both positive and negative clinical outcomes. Children with delayed, broth-only growth of commensal organisms (P. acnes, S. epidermidis, etc.) had the lowest sMAC levels (7.96 ± 1.7 ng/ml), suggesting contamination rather than shunt infection. CONCLUSION. Elevated CSF sMAC levels are both sensitive and specific for diagnosing pyogenic shunt infection and may serve as a useful prognostic biomarker during recovery from infection.