PURPOSE:To examine the validity and reliability of an activity monitor to estimate upper limb activity. METHODS:Thirty-two adults after distal radial fracture were recruited. 15 adults performed five upper limb activities during two testing sessions, one week apart to investigate criterion validity against the criterion reference of three-dimensional motion analysis, convergent validity, and test-retest reliability. 17 adults in two therapy groups wore monitors for three consecutive days at baseline and six weeks post-intervention. Hypothesis testing (noninferiority) assessed comparison of group differences. RESULTS:There were large, significant positive correlations between monitor counts and motion analysis for affected/unaffected wrists during the grocery shelving [r = 0.82, r = 0.73, respectively] and floor sweeping activities [r = 0.54, r = 0.59, respectively]. Large confidence intervals relative to means suggests the monitor could not accurately predict motion analysis distance or acceleration. Relative reliability was excellent in affected/unaffected wrists for crank ergometer [ICC(2,1) = 0.91, ICC(2,1) = 0.88, respectively], grocery shelving [ICC(2,1) = 0.83, ICC(2,1) = 0.89, respectively], and table dusting activities [ICC(2,1) = 0.77, ICC(2,1) = 0.83, respectively]. Correlations and reliability for typing were poor; hypothesis testing of group equivalence was not confirmed. CONCLUSIONS:There is preliminary evidence that an activity monitor is a valid and reliable tool to measure gross arm activity after distal radial fracture. Implications for Rehabilitation Distal radius fractures are one of the most common upper limb fractures that cause activity limitations and participation restrictions. After a distal radius fracture patients are often referred to therapy (physiotherapy, occupational therapy, hand therapy) for rehabilitation to return to pre-injury function and activity levels. Activity monitors may be a valid and reliable measurement tool that therapists can use with their patients to quantify gross arm activity to enable monitoring of injury recovery and rehabilitation adherence. Activity monitors may not be sufficiently reliable and valid to monitor fine movements of the wrist and hand after distal radius fracture.