Background and purpose: We used differential actigraphy as a novel, objective method to quantify motor neglect (a clinical condition whereby patients mimic hemiplegia even in the absence of sensorimotor deficits), whose diagnosis is at present highly subjective, based on the clinical observation of patients' spontaneous motor behavior. Methods: Patients wear wristwatch-like accelerometers, which record spontaneous motor activity of their upper limbs during 24 hours. Asymmetries of motor behavior are then automatically computed offline. On the basis of normal participants' performance, we calculated cutoff scores of left/right motor asymmetry. Results: Differential actigraphy showed contralesional motor neglect in 9 of 35 patients with unilateral strokes, consistent with clinical assessment. An additional patient with clinical signs of motor neglect obtained a borderline asymmetry score. Lesion location in a subgroup of 25 patients was highly variable, suggesting that motor neglect is a heterogenous condition. Conclusions: Differential actigraphy provides an ecological measure of spontaneous motor behavior, and can assess upper limb motricity in an objective and quantitative manner. It thus offers a convenient, cost-effective, and relatively automatized procedure for following-up motor behavior in neurological patients and to assess the effects of rehabilitation.