BACKGROUND: Obesity is a major public health concern requiring innovative interventions that support people to lose weight and keep it off long term. However, weight loss maintenance remains a challenge and is under-researched, particularly in men. The Football Fans in Training (FFIT) programme engages men in weight management through their interest in football, and encourages them to incorporate small, incremental physical activity and dietary changes into daily life to support long-term weight loss maintenance. In 2011/12, a randomised controlled trial (RCT) of FFIT demonstrated effectiveness and cost-effectiveness at 12 months. The current study aimed to investigate long-term maintenance of weight loss, behavioural outcomes and lifetime cost-effectiveness following FFIT. METHODS: A longitudinal cohort study comprised 3.5-year follow-up of the 747 FFIT RCT participants. Men aged 35-65 years, BMI ≥ 28 kg/m2 at RCT baseline who consented to long-term follow-up (n = 665) were invited to participate: those in the FFIT Follow Up Intervention group (FFIT-FU-I) undertook FFIT in 2011 during the RCT; the FFIT Follow Up Comparison group (FFIT-FU-C) undertook FFIT in 2012 under routine (non-research) conditions. The primary outcome was objectively-measured weight loss (from baseline) at 3.5 years. Secondary outcomes included changes in self-reported physical activity and diet at 3.5 years. Cost-effectiveness was estimated at 3.5 years and over participants' lifetime. RESULTS: Of 665 men invited, 488 (73%; 65% of the 747 RCT participants) attended 3.5-year measurements. The FFIT-FU-I group sustained a mean weight loss of 2.90 kg (95% CI 1.78, 4.02; p < 0.001) 3.5 years after starting FFIT; 32.2% (75/233) weighed ≥5% less than baseline. The FFIT-FU-C group had lost 2.71 kg (1.65, 3.77; p < 0.001) at the 3.5-year measurements (2.5 years after starting FFIT); 31.8% (81/255) weighed ≥5% less than baseline. There were significant sustained improvements in self-reported physical activity and diet in both groups. The estimated incremental cost-effectiveness of FFIT was £10,700-£15,300 per QALY gained at 3.5 years, and £1790-£2200 over participants' lifetime. CONCLUSIONS: Participation in FFIT under research and routine conditions leads to long-term weight loss and improvements in physical activity and diet. Investment in FFIT is likely to be cost-effective as part of obesity management strategies in countries where football is popular. TRIAL REGISTRATION: ISRCTN32677491 , 20 October 2011.