We measured some soluble factors in the cerebrospinal fluid (CSF) of patients with adult T-cell leukaemia (ATL) complicated with meningeal infiltration. Interleukin-4 (IL-4) was not detectable in the CSF of all cases with meningeal infiltration. Interleukin-6 (IL-6) was detected in a few patients with ATL. Measurement of IL-4 and IL-6 in CSF had no diagnostic value for meningeal infiltration of ATL cells. Soluble CD4 (sCD4) was highly elevated in all ATL patients with meningeal infiltration. Soluble interleukin-2 receptor (sIL-2R; sCD25) in CSF was markedly elevated in 13/18 ATL patients (72.2%) with meningeal infiltration. Levels of sCD4 and sCD25 in the CSF of ATL patients with central nervous system (CNS) symptoms were significantly higher than those of non-ATL patients with CNS symptoms. These observations indicate that sCD4 and sCD25 in the CSF are probably associated with meningeal infiltration of leukaemia cells that expressed CD4 and CD25 on the surface membrane and new markers for the meningeal infiltration of ATL cells.