The blood-brain barrier (BBB) has a key role in CSF homeostasis and preservation of normal neuronal function. There has been little investigation of changes in barrier function in elderly subjects without evidence of neurological disease. The ageing brain demonstrates increased vulnerability to a variety of insults, which may reflect a deterioration in barrier performance. Direct measurement of BBB function is difficult but the CSF/serum quotients of individual proteins is the most widely used parameter. The present study sought age related changes in the CSF/serum quotients of prealbumin, albumin, immunoglobulin G and alpha 2-macroglobulin in adults with no neurological deficit. A prospective study was established, and 107 subjects (aged 18-89 yrs) were recruited from patients undergoing spinal anaesthesia or diagnostic myelography. A weak but significant positive correlation was found between the CSF/serum quotients and age for all the proteins studied, but with the exception of alpha 2-macroglobulin, these changes probably reflect age related falls in serum protein concentrations found in this study. alpha 2-Macroglobulin appears to be a sensitive indicator of barrier function for large populations, suggesting a subtle decline of BBB integrity with increasing age. This may be a true age-related change or could reflect an increased incidence of subclinical disease in an ageing population.