||In this paper we investigate the potential of a smartphone to measure patients’ changes in physical activity unobtrusively before and after a surgical pain relief intervention. Providing an objective intervention outcome measure to clinicians could enhance subjective assessments from patient questionnaires and contribute to optimal patient treatment. Thus, we show a proof of concept for our smartphone system providing physical activity from acceleration, barometer and location data to infer meaningful activity features that measure the intervention’s outcome. In a case study, we monitored two patients carrying the smartphone 8 days before and another 8 days after a surgical intervention. Results indicate signiﬁcant activity changes before and after intervention while the pain level decreased. Particularly physical activity in the home environment increased signiﬁcantly for both patients where an averaged 98% in crease in walking and a more than 150% gain in fast cadence was measured. Questionnaire assessed activity levels showed no meaningful correlations to activity measurements and turned out to be highly subjective.