AbstractThe objective of the study is to describe the nature of functional limitations in activities and participation in people with Rheumatoid Arthritis (RA) or axial SpondyloArthritis (axSpA) with severe functional disability. Baseline data from people with RA (n = 206) or axSpA (n = 155) and severe functional disability participating in an exercise trial were used. Their three most limited activities were derived from the Patient Specific Complaint (PSC) instrument and linked to the International Classification of Functioning and Health (ICF). The frequencies of ICF categories were calculated and compared with Activities and Participation items of the ICF Core Sets for RA (32 second-level categories) and Ankylosing Spondylitis (AS) (24 second-level categories). In total 618 and 465 PSC activities were linked to 909 (72 unique in total; 25 unique second-level) and 759 (57 unique in total; 23 unique second-level) ICF categories in RA and axSpA. Taking into account all three prioritized activities, the five most frequent limited activities concerned the ICF chapter “Mobility”, and included “Walking” (RA and axSpA 2 categories), “Changing basic body position” (RA and axSpA 1 category), “Stair climbing”(RA) and “Grasping” (RA),“Lifting” (axSpA) and “Maintaining a standing position” (axSpA). In RA, 21/32 (66%) and in axSpA 14/24 (58%) unique second-level categories identified in the prioritized activities are present in the Comprehensive Core Sets. Most limitations of people with RA or axSpA and severe functional disability were seen in the ICF chapter “Mobility”. Most of the identified ICF categories were covered by the corresponding items of the ICF RA and AS Core Sets.