BACKGROUND &AIMS:: Progression from the acute to chronic phase of inflammatory bowel disease cannot be easily evaluated in patients and has not been characterized in animal models. We report a longitudinal study investigating changes in the mucosal immune response in an experimental model of colitis. METHODS:: Severity of colitis, body mass, stool consistency and blood content, serum amyloid A, and tissue histology were examined in interleukin (IL)-10-deficient mice over 35 weeks. The corresponding production of 11-42, IL-18, interferon γ, tumor necrosis factor α, IL-4, and IL-13 by lamina propria mononuclear cells in the inflamed intestine was measured. Administration of neutralizing antibody to IL-12 at distinct times during disease progression permitted evaluation of its therapeutic potential. METHODS:: The clinical manifestations and intestinal inflammation delineated an early phase of colitis (10–24 weeks), characterized by a progressive increase in disease severity, followed by a late phase (>25 weeks), in which chronic inflammation persisted indefinitely. Lamina propria mononuclear cells from mice with early disease synthesized progressively greater quantities of IL-12 and interferon gamma, whereas production of both cytokines dramatically declined and returned to pre-disease levels in the late phase of colitis. Consistent with this pattern, neutralizing antibody to IL-12 reversed early, but not late, disease. In contrast, IL-4 and IL-13 production increased progressively from pre- to early to late disease. CONCLUSIONS:: Colitis that develops in IL-10-deficient mice evolves into 2 distinct phases. IL-12 plays a pivotal role in early colitis, whereas its absence and the synthesis of IL-4 and IL-13 in late disease indicate that other immune mechanisms sustain chronic inflammation.