Raised ventricular CSF myelin basic protein (MBP) concentration has been evidenced in 17 shunted hydrocephalic patients. Contemporary evaluation both from ventricular and lumbar CSF samples showed a concentration ratio of 20:1. In all cases the raised values of ventricular CSF concentration of MBP demonstrated a significant decrease after shunt operation. This preliminary report suggests that this marker is an important index of actual brain damage in hydrocephalus and could be taken in account for the indication of shunt operation.