Distinguishing infected from noninfected ventriculoatrial (VA) or ventriculoperitoneal (VP) shunts is prognostically and therapeutically important. Eighty-seven serum samples from twenty-seven patients with VA or VP shunts were studied for the presence and quantification of circulating immune complexes. Eighty-three percent of the samples from infected shunts presented circulating immune complexes. Mean values of immune complexes in patients with infected shunts were significantly higher than in those without infection. In febrile, septicemic patients with few clinical symptoms, immune complexes were present, and their measurement in serial serum samples was a significant diagnostic aid. If circulating immune complex levels are not detectable, a shunt infection is less likely to be present.