The obstructive tissue in eight malfunctioning ventricular catheters without infection was studied using scanning and transmission electron microscopy. Shunt obstruction was due to debris from ventricular structures such as the choroid plexus and ependymal tissue. There was a preponderance of collagen fibers, and many fibroblasts were present within these tissues. The cytoplasm of the fibroblasts contained extended endoplasmic reticulum. The tissues filled the lumen of the catheters in radially arranged layers. Peeling of the silicone was seen on the surface of the ventricular catheters but was not present in normal silicone catheters. Finger-like microvilli were observed on the free surface of the tissue. Many vessels were seen in thtie transverse section of the tissue. Activated fibroblasts and vascularization may be important in tissue growth in ventricular catheters.