Obesity is regarded as a chronic disease with a need for long term support. Hence this study was designed to evaluate a web-based aftercare (WBI), following a face-to-face lifestyle intervention for obese women. The superiority of the WBI towards a printed manual aftercare intervention (PMI) regarding increase of nutrition and exercise self-efficacy was expected. It was also predicted, that the WBI would reduce the dropout rate and advance sustainability compared to the PMI and a historical control group (CG). To evaluate the efficacy of this web-based approach, a two armed randomized trial was designed. First, both intervention groups received the same face-to-face lifestyle treatment. This included a 4 month "introduction phase" and (after allocation into intervention groups) and another 2 month "training phase" with a follow-up tool supplementing the group sessions. The 6 month "self-monitoring phase" was supported by a web-based aftercare intervention (WBI) or a printed manual aftercare intervention (PMI). Results of the present study did not verify superiority of the WBI over the PMI - changes in values of exercise or nutrition self-efficacy between the groups were not significant. Feedback on the web-based intervention though, showed a slightly higher percentage in acceptability (content with allocation to intervention group: WBI =81% PMI = 55% p = .074). Another important finding of the present study was, that keeping in touch with the program supervisor proved to be easier for participants of the WBI (always/often: WBI = 20% PMI= 4.8%, sometimes: WBI = 45% PMI = 23.8% p = .054). Subgroup analyses for engagement in the aftercare intervention did not show significant differences between the groups, but indicated a benefit of participation in the follow-up support (slightly higher values in exercise and nutrition self-efficacy scales for users of both the WBI and PMI). Regarding sustainability, in this study the measured anthropometric data at the end of the group sessions could be maintained within the predefined range until the final evaluation at 12 months. None of the groups proved superiority over the others (BMImonth: WBI = + .07 kg/m2 PMI = - .02 kg/m2 CG = + .03kg/m2 p = .465; body fat%month: WBI = + .05% PMI = + .03% p = .916).   Analysis of attrition indicated, that the major part of the participants (study group = 80% CG = 72.4%) who droped out of the preset study, left the program during the first 6 months of group sessions - before the start of any aftercare intervention. Only 20% of the WBI and the PMI left the program during the aftercare intervention (CG = 27.6%). Calculations for dropouts (DO) and completers (Comp) between 6 to 12 months showed no significant differences between the groups (DOWBI = 12.5% CompWBI =87.5% DOPMI =15.4% CompPMI =84.6% DOCG = 19.8% CompCG =82.4% p=.763). It can be concluded, that in this study, the web-based aftercare did not show significant superiority (in self-efficacy changes, maintenance of anthropometric parameters or dropout rates) over the printed manual support group or the control group. Still, this form of intervention was well accepted and beneficial in stabilizing or improving scores of questionnaires for participants engaging in the support tool. The results encourage efforts to further improve current treatment designs and complement standard care with technical solutions, to make engagement in an intervention more desirable.