Using an enzyme-linked immunosorbent assay (ELISA), we measured the levels of neuron-specific enolase (NSE) in the sera of 100 normal controls, 31 patients with adult T cell leukaemia (ATL), 13 patients with acute lymphoblastic leukaemia (ALL), 19 patients with acute non-lymphoblastic leukaemia (ANLL) and 17 patients with human T lymphotropic type I (HTLV-I) associated myelopathy (HAM). Levels higher than 5 ng/ml were considered as positive for NSE, since this value was almost 2 standard deviations (S.D) above the mean of normal controls. The rates of positive NSE levels were 55 percent in patients with ATL, 54 percent in patients with ALL, 11 percent in patients with ANLL and 0 percent in patients with HAM. High NSE levels fell to near normal levels in patients with ATL and ALL responding to cytotoxic therapy. It is concluded from these results that NSE may be a useful tumour marker for lymphoid malignancies.