The region-based segmentation paradigm is a well known technique for image segmentation. In the¯rst part of this work the robustness of region-based algorithms is studied. It is shown that within a small parameter range, which leads to good segmentation results in the majority of cases, bad segmentation results may occur. In fact, such local instability is a problem of regionbased methods and reasons for its occurrence are discussed. In the second part of the work, an ensemble solution for this problem based on the median concept is proposed. Two variants, set median and generalized median, are presented and experimentally compared. Extensive experimental results demonstrate the potential of the proposed median approach for solving the instability problem.