Background and aims: Despite promising results, only a few studies have been published on serum calprotectin as a biomarker in IBD. Recently, plasma measurements of calprotectin have been shown to be more reliable than serum measurements. In this study, we aim to assess plasma and serum calprotectin measurements as biomarkers of disease activity in paediatric and adult ulcerative colitis.Methods: Paediatric (5-18 years) and adult (>18 years) patients scheduled for colonoscopy due to suspected or confirmed ulcerative colitis were included prospectively. Stool and blood samples were collected at time of colonoscopy and patient symptom scores were recorded. At colonoscopy the Ulcerative Colitis Endoscopic Index of Severity was recorded. Histology was graded according to the Geboes score.Results: 84 patients where included; 30 paediatric and 54 adult patients. Plasma calprotectin had a stronger correlation to all outcome variables than serum calprotectin. Plasma calprotectin correlated positively to disease extent (Rho = 0.53, p < .0001), symptoms scores (Rho = 0.54, p = .002, only in the paediatric cohort), endoscopic scores (Rho = 0.39, p = .0003), histological scores (Rho 0.28, p = .01) and, when using endoscopic assessment of severity as reference, could discriminate active disease from patients in remission (p = .03).Conclusions: While more studies are needed to assess if plasma calprotectin can discriminate healthy individuals from ulcerative colitis, this study indicates that plasma calprotectin can be used as a biomarker of disease activity, especially in cases where faecal calprotectin measurements are cumbersome either due to patient compliance or logistical requirements.