Validated clinical scales of functional dexterity of stroke patients provide only sparse and controlled observations of the neurorehabilitation process. Continuous naturalistic monitoring the progress during the rehabilitation therapy of outpatients remains unsatisfactorily resolved. We propose a new indicator of functional performance capitalizing on smart devices and the cloud services. The new index of limb usage balance (ILUB) derived from accelerometry data, provides an objective scalar that it is easy to estimate and interpret, and adequate for continuous uncontrolled monitoring. Three studies for the verification and validation of the index and accompanying platform are presented. We show that the index is unbiased to handedness dominance and provide preliminary evidence that it is appropriate for patients even during the subacute stage. The new index can complement clinical scales to observe the patient progress. CCS Concepts • Social and professional topics → Medical technologies; • Human-centered computing → Ubiquitous and mobile computing; Ubiquitous and mobile computing systems and tools; • Applied computing → Health care information systems.