Background and aims Diagnosis and monitoring of ulcerative colitis (UC) includes conventional colonoscopy. This procedure is invasive and does not exclude small-bowel Crohn's disease (CD). Current therapeutic goals include mucosal healing which may lead to an increased number of endoscopic procedures in many patients. The small-bowel colon capsule endoscopy (SBC-CE) system visualizes the small bowel and colon. The aim of this study was to evaluate the performance and adverse events of SBC-CE in patients with UC. Methods This was a prospective, feasibility study involving two study sites. Patients with active UC underwent SBC-CE and colonoscopy. Kappa statistics were performed to assess the agreement between SBC-CE and colonoscopy. Adverse events (AEs) data were collected throughout and following the procedure. Results In total, 30 consecutive patients were recruited, and 23 of those were included in the final analysis. For the primary end point, evaluation of the extent of UC disease in the colon, the percent agreement between SBC-CE and colonoscopy was moderate (56.5 %); kappa coefficient 0.42. The percent agreement between SBC-CE and colonoscopy for UC disease activity, based on Mayo endoscopic sub-score, was 95.7 %; kappa coefficient 0.86. Disease activity in the more proximal small bowel was detected in two patients with SBC-CE. No SBC-CE device-related AEs were reported. Conclusions When comparing SBC-CE to conventional colonoscopy, there was a moderate agreement for the extent of UC disease and a very good overall agreement between the two modalities for UC disease activity.