Objectives: To construct a single indicator on impairment level for lower extremity Complex Regional Pain syndrome type I (CRPS I). Design: The Impairment level SumScore (ISS) for upper extremity CRPS I was adapted to be used for lower extremity evaluation. Medline literature search and research findings were used to adapt the upper extremity version of the ISS, with emphasis on reliability, responsiveness and validity of measurement instruments. Where needed, additional patient data was gathered to evaluate these aspects for different measurement instruments. Setting: An outpatient clinic of a university hospital in the Netherlands. Participants: Two groups consisting of 17 and 26 healthy volunteers, and two groups of respectively 40 and 18 lower extremity CRPS I patients according to Veldman's criteria. Main outcome measures: VAS and McGill pain scores, water displacement volumeter values, and physicians' and patients' assessment of CRPS I severity. Results: A combination of measurements, incorporating pain (VAS and McGill), temperature (infrared thermometer), volume (water displacement volumeter) and active range of motion (universal goniometer), was converted in a single score ranging from 5 to 50. The reliability, as well as the responsiveness was adequate. Conclusions: The lower extremity ISS permits evaluation of the most prominent symptoms in CRPS I, and can be used to monitor changes in CRPS I.