The one-year outcome of the randomized controlled T.A.F.F. (Telephone based Adiposity prevention For Families) study is presented. Screening of overweight (BMI-SDS > 90th centile) children 3.5-17.4 years was performed via the German CrescNet database, and candidates were randomized to an intervention group (IG) and control group (CG). The intervention consisted of computer-aided telephone counselling for one year, supported by mailed newsletters. The primary endpoint was change in BMI-SDS; secondary endpoints were eating behavior, physical activity, media consumption, quality of life. Data from 289 families (145 IG (51% females); 144 CG (50% females)) were analyzed (Full Analysis Set: FAS; Per Protocol Set: PPS). Successful intervention was defined as decrease in BMI-SDS ≥ 0.2. In the FAS, 21% of the IG was successful as compared to 16% from the CG (95% CI for this difference: (-4, 14), p = 0.3, mean change in BMI-SDS: -0.02 for IG vs. 0.02 for CG; p = 0.4). According to the PPS, however, the success rate was 35% in the IG compared to 19% in the CG (mean change in BMI-SDS: -0.09 for IG vs. 0.02 for CG; p = 0.03). Scores for eating patterns (p = 0.01), media consumption (p = 0.007), physical activity (p = 9 × 10-9), quality of life (p = 5 × 10-8) decreased with age, independent of group or change in BMI-SDS. We conclude that a telephone-based obesity prevention program suffers from well-known high attrition rates so that its effectiveness could only be shown in those who adhered to completion. The connection between lifestyle and weight status is not simple and requires further research to better understand.