Introduction: The incidence of early-onset inflammatory bowel disease is increasing in Japan. Objective: This study aimed to analyze the treatment and progress of early-onset inflammatory bowel disease. Methods: This prospective survey evaluated the data of 43 patients aged <8 years who were diagnosed with inflammatory bowel disease (IBD) from the time of diagnosis to 36 months after registration. Results: A total of 12 patients with Crohn’s disease (CD), 21 with ulcerative colitis (UC), and 3 with unclassified IBD were enrolled. The mean disease onset age was 3 years and 7 months. Colon and anal lesions were present in 100 and 50% of patients with CD, respectively. Granulomas were detected in 5 patients (41.7%). Dietary elimination including elemental diet was performed in all patients. Eleven patients (91.7%) were in remission by initial induction therapy, and 72.7% maintained remission for 36 months. Three patients (14.3%) with UC had familial history, 71.4% had pancolitis-type UC, and 66.7% exhibited disease of moderate severity. Colectomy was performed in 4 patients (21.1%). Eighteen patients (85.7%) were in remission by initial induction therapy; however, only 15.8% maintained remission for 36 months. Anal complication was more prevalent in infantile-onset IBD than in childhood-onset IBD (p = 0.014). Conclusions: Among Japanese patients aged <8 years who were diagnosed with IBD, colitis-type disease was more common in CD and pancolitis was more common in UC. As the courses of several patients were severe, identifying primary immunodeficiency appears to be necessary to confirm background disease.