Serum and cerebrospinal fluid (CSF) concentrations of interleukin (IL)-6, IL-8, IL-10, tumour necrosis factor-alpha and interferon-gamma were determined in 46 Trypanosoma brucei gambiense sleeping sickness patients in DR Congo, before and after treatment. According to their CSF cell number before treatment, patients were classified as early-stage (0-5 cells/microL), intermediate-stage (6-20 cells/microL) or late-stage patients (> 20 cells/microL). In serum, slightly higher IL-8 concentrations were found in early-stage patients compared to intermediate- or late-stage patients. These high IL-8 levels dropped after treatment. Higher IL-10 concentrations were detected in serum of patients in intermediate or late stage compared to early-stage patients. In both intermediate- and late-stage groups, serum IL-10 decreased after treatment. In CSF, elevated concentrations of IL-6, IL-8 and especially of IL-10 were observed in late-stage T. b. gambiense patients. After treatment, these concentrations dropped to levels similar to those of the other patients. Tumour necrosis factor-alpha was detected only in a few serum and CSF samples, which were scattered over the different patient groups. Interferon-gamma was detected in serum of 5 patients and remained undetectable in CSF.