A 28-year-old man presented with hydrocephalus due to an infection by Cryptococcus neoformans involving his ventriculoatrial shunt. Ultrastructural studies of the ventriculoatrial shunt demonstrated yeastlike organisms consistent with C. neoformans within a biofilm on the appliance. Quantitative microbiological studies of segments of the shunt demonstrated C. neoformans in a concentration gradient from 9 X 10(6) colony-forming units (CFU)/0.5-cm section involving the ventricular portion of the catheter to 1 X 10(2) CFU/0.5-cm section at the vascular tip. The clinical, microbiological, and ultrastructural data suggest that this cryptococcal infection started as a ventriculitis or encephalitis with extension to the meningeal surface. Future application of these methods may further elucidate the pathogenesis of ventriculoatrial shunt infections.