A continuous ambulatory activity monitor allows objective measurement of the amount and intensity of physical activity. We examined the reliability and validity of this device in the assessment of seven aspects of function over a period of 24 hours in 20 patients who had undergone limb salvage or amputation for a tumour in the leg. The test-retest reliability was determined by undertaking identical assessments on two separate days. The measurements were compared with other indicators of functional status and quality of life in order to determine the validity of the monitor. Its reliability was satisfactory, with intraclass correlation coefficients ranging from 0.65 to 0.91. Significant correlations were seen between the ‘time spent walking’ and the Musculoskeletal Tumor Society rating scales and the Rand-36 physical functioning score. There was also a significant association between the ‘movement intensity during walking’ and the Musculoskeletal Tumor Society score. The satisfactory reliability and validity of the monitor shows considerable promise for its use as a device for measuring physical activity objectively in patients after surgery for limb-salvage or an amputation