Plantar fasciitis is the most common cause of heel pain. Diverse non-operative treatment options are available. The purpose of this study was to determine if a single platelet-rich-plasma injection at the origin of the plantar fascia in patients with plantar fasciitis gives a functional improvement. Patients with plantar fasciitis and failed conservative treatment were included in this retrospective study. Included patients were sent four questionnaires after platelet-rich-plasma injection. Primary outcome is functional improvement, determined by foot function index in which lower scores correlates with a better foot function. A total of 61 feet in 58 patients were included. The median foot function index before treatment was 69.4 and after treatment 31.8, which is a significant decrease. In 80.3% of the patients the foot function index decreased. Therefore platelet-rich-plasma injection seems to be effective in treatment of patients with plantar fasciitis when conservative treatment failed.