Particularly in its empiric part this report is based on model tests carried out by universities and manufacturers of hydraulic turbo-machines, who are members of Working Group 5 (WG 5) of the "Section on Hydraulic Machinery, Equipment and Cavitation" of the IAHR. The scale-up behaviour is dealt with on the basis of separate analyses of losses, whereby an important section refers to the influence of surface machining and depth of roughnessRtof separate machine components (full-size plant). Corresponding calculations result in figures which express the modifications in efficiency Δη(ri)due to the accepted roughness, size of the machine and specific energyYof the head. The figures forRtrefer to data supplied by leading manufacturers of hydraulic equipment for power stations. Apart from a differentiated insight into the scaling-up process, the report also provides indications and guide-lines for answering the question as to the economically acceptable amount of machining for which the losses caused by roughness remain within reasonable, predetermined limits. This corresponds with the ende avour to realize optimum machining of machine components in contact with the flow, namely in relation to the type of machine, its size (D) and specific energy (Y). The final result is a global efficiency scale-up Δη(tot), which takes into account, apart from the influence of the Reynolds number, also the increase in losses due to the roughness of the surfaces of the full-size machine. The method of calculation is explained with the aid of tables, taking as an example a Francis turbine (nq^≈ 45), whilst diagrams illustrate the influence of D, Y and Rt on Δη(ri)and Δη(tot).