In 24 patients with cutaneous T-cell lymphomas (CTCL) the clinical course was documented by determination of a tumour burden index (TBI) on entering the study and six months later. In addition to the first TBI, soluble interleukin-2 receptor levels (slL-2R) in the serum were determined using an ELISA technique in 23 patients. In 18 patients natural killer cell (NK) activity was assessed by a 4 h chromium-51 release assay. Statistical analysis revealed a negative correlation between NK activity and an increase of TBI as well as NK activity and slL-2R. Furthermore, there was a strong positive correlation between the increase of TBI and slL-2R. slL-2R and NK activity might be prognostic factors in CTCL patients.