This is an observational study of a prospective cohort of guselkumab-initiating Crohn's Disease (CD) and Ulcerative Colitis (UC) patients to identify new or incident spondyloarthritis. This study will include adult patients aged 18-75 with IBD who are undergoing guselkumab intravenous or subcutaneous induction over a 1-year timeframe.

Inclusion Criteria:

• Confirmed diagnosis of Crohn's disease or Ulcerative Colitis disease based on confirmatory colonoscopy with biopsies, or if small bowel only, MRE with corresponding history
• Age between 18 to 75 years old
• Inflammatory bowel disease unclassified (IBD-U) subjects are eligible for inclusion if based on investigator assessment, the subject meets criteria for IBD-U, defined as chronic inflammatory bowel disease with overlapping features of Crohn's disease or ulcerative colitis
• Initiating Guselkumab therapy in the next 30 days
• Bio-naïve or bio-experienced* to ≤3 mechanisms of action *Defined as prior exposure or failure, as defined by inadequate efficacy or intolerance

Exclusion Criteria:

• Unable to provide consent for participation
• Known SpA diagnosis, prior to screening
• Has started Guselkumab therapy
• Prior exposure to IL-23 therapy (like Mirikizumab, Ustekinumab, etc.)
• Prior exposure to TB, active or currently undergoing treatment for latent TB
• Pregnancy as some pregnancy symptoms could be mistaken for SpA symptoms
• Prior exposure to combination advanced biologic therapies

CD or UC patients initiating guselkumab.

Data will be shared only with funder (Janssen).