BACKGROUND Nifurtimox is a 5-nitrofuran derived antiprotozoal drug used to treat diseases caused by trypanosomes including Chagas' disease and sleeping sickness (African trypanosomiasis). Available methods for the determination of nifurtimox in plasma are tedious and of low sensitivity. For the first time, an isotope dilution HPLC/MS/MS method for the sensitive quantitation of nifurtimox down to 10.0 μg/l in plasma is described. RESULTS Protein precipitation was used for sample preparation. Samples were analyzed on a standard triple quadrupole tandem mass spectrometer. The validated concentration range covers 10.0 μg/l (LLOQ) to 5000 μg/l. Interassay accuracy and precision (%CV) ranged from 98.4 to 101%, and 2.61 to 10.1%, respectively. CONCLUSIONS The method consists of very simple sample preparation and provides unmatched sensitivity, high reproducibility and robustness enabling analysis of large sample numbers. Method performance met current guidelines on bioanalytical method validation.