Walking ability was assessed in twenty patients before and one year after knee replacement with a cemented unicompartmental, Brigham prosthesis (mean age 63.4 years, nine women). All patients had moderate medial gonarthrosis. One year after surgery, knee function, assessed by the BOA score, self-selected and maximal walking speed as well as single limb support of the involved leg were increased. Pain and exertion during walking and oxygen cost of level walking were decreased at all measured speeds. Individual improvement in self-selected walking speed was correlated to improvement in maximal walking speed. Individual decrease of oxygen cost of level walking was correlated to decrease of perceived pain and exertion during walking. For clinical routine purpose clinical assessment, especially of pain, supplemented with measurement of self-selected walking speed were found to be sufficient for assessing effects of treatment such as unicompartmental prosthetic knee replacement.