Experimental work carried out in this work has investigated the scale-up of microbial fuel cell (MFC) technology by studying the stacking of single microbial fuel cells, paying attention to the electric and hydraulic connections between each unit. To do this, the performance of three stacks (which were set up with different configurations) was studied for more than three months. The first stack (two hydraulically non-connected cells) was operated for 80 days without any electric connection between them, in order to determine the reproducibility of the performance of a single MFC, and then it was electrically connected in parallel for 20 days to determine if the electricity produced by each single cell was added when they were joined in the stack. The other two stacks (with five and ten cells, hydraulically connected) were connected electrically in series during the first 80 days and in parallel during the last 20 days. The results confirmed that connection in parallel allows higher current intensities and power to be obtained, and that the total electrode surface area attained with the stack is directly related to the production of electricity and to the removal of COD, although not in a linear way.