Introduction: Given the lack of data, we aimed to assess the impact of the length of diagnostic delay on the natural his- tory of ulcerative colitis (UC) in pediatric (diagnosed <18 years) and adult patients (diagnosed ≥18 years). Methods: Data from the Swiss Inflammatory Bowel Disease Cohort Study were analyzed. Diagnostic delay was defined as the interval between the first appearance of UC-related symp- toms until diagnosis. Logistic regression modeling evaluat- ed the appearance of the following complications in the long term according to the length of diagnostic delay: co- lonic dysplasia, colorectal cancer, UC-related hospitalization, colectomy, and extraintestinal manifestations (EIMs). Re- sults: A total of 184 pediatric and 846 adult patients were included. The median diagnostic delay was 4 [IQR 2–7.5] months for the pediatric-onset group and 3 [IQR 2–10] months for the adult-onset group (p = 0.873). In both, pedi- atric- and adult-onset groups, the length of diagnostic delay at UC diagnosis was not associated with colectomy, UC-re- lated hospitalization, colon dysplasia, and colorectal cancer. EIMs were significantly more prevalent at UC diagnosis in the adult-onset group with long diagnostic delay than in the adult-onset group with short diagnostic delay (p = 0.022). In the long term, the length of diagnostic delay was associated in the adult-onset group with colorectal dysplasia (p = 0.023), EIMs (p < 0.001), and more specifically arthritis/arthralgias.