Eflornithine (difluoromethylornithine, DFMO) has recently been approved for the treatment of Trypanosoma brucei gambiense trypanosomiasis. Treatment failures have been infrequent but have occurred among patients treated with oral DFMO only, and among children. To investigate the higher frequency of failures observed in young patients, DFMO trough concentrations in serum and cerebrospinal fluid (CSF) were measured at the end of treatment in 13 children and 50 adults who had received 200 mg/kg intravenously every 12 h for 14 d. Mean DFMO concentration in CSF was significantly lower among children aged less than 12 years when compared to older patients (25.1 vs 68.9 nmol/mL, P < 0.001). Mean serum concentration was also lower in children (49.2 vs 87.5 nmol/mL, P = 0.03). Among patients who received DFMO as initial therapy for sleeping sickness, the mean CSF/serum ratio was lower in children (0.41 vs 0.91, P < 0.005). The 3 patients who failed DFMO treatment had CSF trough concentrations around or below 50 nmol/mL. Convulsions and anaemia were associated with higher drug levels and previous therapy with melarsoprol. The lower CSF drug concentrations observed in children could result from higher renal clearance and different CSF pharmacokinetics of DFMO in that age group. To avoid treatment failures, a 6-hourly regimen as well as higher DFMO dosage based on body surface area rather than on weight are recommended for children.