The purpose is to compare the effectiveness and imaging changes (US and MRI) between PRP and corticoids injections for the treatment of chronic plantar fasciitis, using clinical results evaluated by the visual analogue scale (VAS), the AOFAS clinical rating system and the modified Roles and Maudsley score, and using imaging results (US and MRI). Our hypothesis is that PRP infiltrations are a more effective therapeutic method than infiltrations with corticosteroids. A single-centre, non randomized, prospective study of 40 consecutive patients (40 feet) with plantar fasciitis who had not responded to conservative treatment for at least 6 months was undertaken. The first 20 consecutive patients (group A) were treated with two local injections of 4ml of a PRP concentrate. The second group of 20 patients (group B) were injected with 4ml of 40mg methylprednisolone. Clinical results were evaluated using a visual analogue scale (VAS), the AOFAS clinical rating system and the modified Roles and Maudsley score, with a mean follow-up of 33 months. Imaging results were evaluated by plantar US after 3 and 6 months, and MRI after 6 months. There were no complications arising from the treatment. In group A (PRP), the VAS changed from 8.25 to 1.85 and the AOFAS from 47.05 to 92.10. In group B (methylprednisolone), the VAS changed from 7.7 to 5.30 points and from 50.85 to 49.75 on the AOFAS. In the imaging tests, the thickness of the fascia in group A changed from 7.90mm to 4.82mm over 3 months following the injection, maintaining this thickness in the biannual controls. In group B the change was from 8.05mm to 6.13mm over 3 months, increasing to 6.9mm after 6 months. The other inflammatory signs improved in all cases, especially in group A. The treatment of chronic plantar fasciitis by two injections of PRP is a safe, more efficient and long-lasting method than corticoid injections.