Interleukin (IL)-2 has well-recognized effects on cerebral endothelial cells and, therefore, may mediate disruption of the blood-brain barrier in patients with multiple sclerosis (MS). To evaluate the in vivo relationship of the IL-2 system to blood-brain barrier impairment in MS, levels of IL-2 and soluble IL-2 receptors (sIL-2R) in cerebrospinal fluid (CSF) and serum samples from 50 patients with active MS and 49 controls were correlated with values of the CSF to serum albumin ratio. Intrathecal levels of IL-2 and sIL-2R were significantly higher in MS compared with the control groups and correlated with albumin ratios in MS patients. Intrathecal levels of IL-2 and sIL-2R also correlated with the degree of barrier damage in these patients. It is suggested that intrathecal levels of IL-2 and sIL-2R are related to barrier impairment in MS and may be important in understanding some of the pathological changes of this condition.