Whilst deep brain stimulation of the subthalamic nucleus (DBS-STN) improves the motor symptoms of Parkinson's disease (PD), its effect on daily activity is unknown. We aimed to quantify changes in ambulatory activity following DBS-STN in advanced PD using novel accelerometry based measures that describe changes to the volume and pattern of walking. Seventeen participants with advanced PD were measured over a 7-day period using an activPAL (™) activity monitor. Data were collected 6 weeks before and 6 months after surgery and included measures that describe the volume and pattern of ambulatory activity (number of steps per day, accumulation, diversity and variability of walking time), alongside standard measures for disease severity, freezing of gait, gait speed, and extended activities of daily living. Activity outcomes were compared pre- and 6 months post-surgery using linear mixed models and correlated with standard outcomes. The results of this study are despite significant improvements in motor symptoms after surgery, the volume of ambulatory activity (total number of steps per day) did not change (P = 0.468). However, significant increases in length and variability of walking bouts emerged, suggesting improvements in diversity and flexibility of walking patterns. Motor severity and extended activities of daily living scores were significantly correlated with walking bout variability but not with volume of walking. Thus, the conclusions are reduction in motor symptom severity after DBS-STN translated into selective improvements in daily activity. Novel measures derived from accelerometry provide a discrete measure of performance and allow closer interpretation of the impact of DBS-STN on real-world activity.