The presence of complex I, or NADH dehydrogenase, in Trypanosomatidae is debated. Several subunits of complex I have been identified by biochemical studies, but the overall composition of the complex has remained elusive. Here, the authors review the present literature related to this mitochondrial activity and carry out a bioinformatic analysis to allow the prediction of the composition of a putative trypanosomatid complex I. The complex comprises at least 19 subunits and has a minimum mass of 660 kDa. It is larger than the corresponding bacterial enzyme but smaller than the typical mitochondrial enzyme of eukaryotes. All subunits known to be involved in electron transport are present, but the complex does not seem to be involved in energy transduction because four membrane subunits, normally encoded by the mitochondrial genome and supposed to be involved in proton extrusion, are missing.