One of the major advances in the understanding of inflammatory bowel disease has been the observation that mice with immunoregulatory defects, such as interleukin‐2 knockout (IL‐2 –/–) mice, develop spontaneous gut inflammation. Here we have characterized the immune response in the ileum, caecum and colon of these mice before and after the onset of colitis by examining the cellular infiltrate, the cytokines produced by these cells and the mucosal vascular addressin MAdCAM‐1. IL‐2 –/– mice developed colitis after 35 days of age and before this the mice were apparently healthy. IL‐2 –/– mice aged over 35 days with colitis had large numbers of CD4⁺, CD8⁺, αβ T‐cell receptor (TCR)⁺ and γδ TCR⁺ T cells, macrophages, dendritic cells and MAdCAM‐1⁺ endothelial cells in the caecum and colon. This was associated with an increase in the number of interferon‐γ (IFN‐γ), IL‐1 and tumour necrosis factor‐α (TNF‐α) transcripts and a decrease in IL‐4 and IL‐10 transcripts. Treatment of IL‐2 –/– mice with cyclosporin A significantly delayed mortality. Interestingly, IL‐2 –/– mice under 35 days, although healthy, did show some subtle immunological signs of preclinical disease. There was a significant increase in the number of macrophages and dendritic cells in the colonic lamina propria and increased mRNA for IL‐1 and TNF‐α. There were also increased numbers of MAdCAM‐1⁺ endothelial cells, but IFN‐γ transcripts were not elevated. These results suggest that T‐cell‐mediated colitis in IL‐2 –/– mice may be secondary to an initial non‐specific inflammation.