We tested the performance, acceptance and user satisfaction of a tool to stimulate physical activity. The tool consisted of an accelerometer, a smartphone app and a server/web application. Patients received feedback concerning their physical activity relative to a goal, which was set in dialogue with their practice nurse. Nurses could monitor their patients' physical activity via a website. Twenty patients with COPD or type 2 diabetes used the tool for three months, combined with behaviour change counselling. Physical activity data were collected at the server and a log file was used to record technical problems. We interviewed patients and nurses after every consultation. At baseline, and after the intervention, patients completed questionnaires. Participants were positive about the tool, although motivation dropped when technical problems occurred caused by log-in and connectivity errors. On average, physical activity increased from 29 (SD 21) min per day in the first two weeks to 39 (SD 24) min per day in the last two weeks (P = 0.02), and quality of life scores increased from 0.76 (SD 0.21) to 0.84 (SD 0.17) (P = 0.04). Provided that no connectivity problems occur, the tool is a feasible intervention when embedded in primary care, and has a positive effect on physical activity levels.