OBJECTIVE: To examine an intensive lifestyle intervention (ILI) compared with diabetes support and education (DSE) on 4-year change in fitness and physical activity (PA), and to examine the effect of change in fitness and PA, adjusting for potential confounders, on glycemic control in the Look AHEAD Trial. RESEARCH DESIGN AND METHODS: Subjects were overweight/obese adults with type 2 diabetes mellitus (T2DM) with available fitness data at 4 years (n = 3,942).This clinical trial randomized subjects to DSE or ILI. DSE subjects received standard care plus information related to diet, PA, and social support three times per year. ILI subjects received weekly intervention contact for 6 months, which was reduced over the 4-year period, and were prescribed diet and PA. Measures included weight, fitness, PA, and HbA1c. RESULTS: The difference in percent fitness change between ILI and DSE at 4 years was significant after adjustment for baseline fitness and change in weight (3.70 vs. 0.94%; P < 0.01). At 4 years, PA increased by 348 (1,562) kcal/week in ILI vs. 105 (1,309) kcal/week in DSE (P < 0.01). Fitness change at 4 years was inversely related to change in HbA1c after adjustment for clinical site, treatment, baseline HbA1c, prescribed diabetes medication, baseline fitness, and weight change (P < 0.01). Change in PA was not related to change in HbA1c. CONCLUSIONS: A 4-year ILI increased fitness and PA in overweight/obese individuals with T2DM. Change in fitness was associated with improvements in glycemic control, which provides support for interventions to improve fitness in adults with T2DM.