INTRODUCTION: Gait impairment and motor-cognitive interaction are key features of Parkinson's disease (PD) and critically affect functional autonomy and fall risk. Wearable inertial sensors allow for objective assessment of gait performance, while the Ф-bonacci gait number has been shown to provide a composite gait performance measure integrating self-similarity, symmetry, and double-support consistency. Building on previous cross-sectional findings, this observational pilot study represents a longitudinal follow-up that extends the application of the Ф-bonacci index framework towards the monitoring of walking performance progression over time. Indeed, this pilot study investigated changes in gait performance and motor-cognitive interaction in people with PD, using conventional clinical scales and the Ф-bonacci gait number. METHODS: Nineteen individuals with PD - carrying out conventional outpatient multidisciplinary rehabilitation - underwent clinical and instrumented gait assessments at baseline (T0) and one-month later (T1). Gait data were collected during the 6-Minute Walk Test using wearable inertial sensors. The changes in spatiotemporal parameters, the Ф-bonacci gait number and its components, and the dual-task performance (Timed Up and Go Dual Task, TUG-DT) were analyzed, together with associations between gait harmonicity, clinical motor improvement, and dual-task performance. RESULTS AND DISCUSSION: Participants at T1 showed significant improvements in functional mobility and spatiotemporal gait parameters, along with a reduction in the Ф-bonacci gait number, mainly driven by enhanced gait self-similarity and double-support consistency. Such improvements in gait harmonicity were significantly associated with both clinical motor improvement and enhanced dual-task performance, while global cognitive scores remained stable. These findings support the claim that Ф-bonacci gait number is a longitudinal marker sensitive to harmonic gait performance in PD and suggest a condition-dependent motor-cognitive coupling in the presence of dual-task demands.