The study aimed to assess the characteristics and reported behaviour change of participants in the BBC's `Fighting Fat, Fighting Fit' (FFFF) campaign. A postal questionnaire survey was sent to a random sample of 6000 adults registering with the FFFF campaign at the start of the campaign and 5 months later. Demographic characteristics, weight, eating behaviour and activity patterns were assessed. In total, 3661 respondents completed the baseline questionnaire and 2112 (58%) of these completed a follow-up evaluation questionnaire 5 months later. The majority of evaluation participants were women and classified as `overweight' or `obese'. Participants reported significant reductions in weight, and in fat and snack intake, and significant increases in exercise levels, and in fruit, vegetable and starch intake during the 6 months of the campaign. These effects remained significant if non-responders were assumed to have made no change. These results show that mass-media campaigns might make a contribution to weight control at the population level, but particular subgroups such as men and people under 25 may require specifically targeted campaigns. In addition, whilst such campaigns may initially attract obese people, they may be more likely to drop out of the campaign than overweight and normal weight individuals.