Methods: This study (IMPAACT) is a prospective, cohort study including 30 PsA patients (fulfilling the the Classification Criteria for Psoriatic Arthritis (CASPAR criteria)) with pain at the Achilles tendon insertion. At baseline, all participants will be examined clinically regarding overall PsA disease activity (Psoriasis Area Severity Index (PASI skin score), the Research Consortium of Canada (SPARCC) Enthesitis score, swollen/tender joint count) and asked to fill out a questionnaire consisting of the validated Scandinavian (Danish) version of the Victorian Institute of Sport Assessment of Achilles tendons (VISA-A) in addition to a study-composed questionnaire comprising general questions regarding patient characteristics. An ultrasonic examination of the Achilles tendons will be performed before ultrasound guided Achilles tendon specimens will be obtained from the most painful Achilles tendon. Tendon samples will be evaluated immunohistochemically by quantifying the presence of macrophages (CD68-KP1+), T-lymphocytes (CD3+), B-lymphocytes (CD20+), natural killer cells (CD56+), neutrophils (granzyme-B+), mast cells and inflammatory markers using a stereological technique. Also, a protein analysis of the tendon tissue will be conducted. Venous blood will be analysed for levels of systemic inflammatory markers, as well as screened for cardiovascular risk factors including dyslipidaemia, and diabetes. After the baseline examination, all participants will be treated and monitored in accordance with the Danish national guideline recommendations for PsA patients. A 3-months follow-up examination will be performed to determine the remission status of the Achilles tendon pain. Data will be analysed using the STATA statistical package (version 12; StataCorp LP).