Overfitting is a significant challenge in the application of Multiple Instance Learning (MIL) methods for Whole Slide Image (WSI) analysis. Visualizing attention heatmaps reveals that current MIL methods focus on a subset of discriminative instances, hindering effective model generalization. To tackle this, we propose Attention-Challenging MIL (ACMIL), aimed at forcing the attention mechanism to focus on more challenging instances. ACMIL incorporates two techniques, Multiple Branch Attention (MBA) to capture more discriminative instances and Stochastic Top-K Instance Masking (STKIM) to suppress top-k salient instances. Evaluation on three WSI datasets with two pre-trained backbones outperforms state-of-the-art methods. Additionally, through heatmap visualization and UMAP visualization, this paper comprehensively illustrates ACMIL's effectiveness in overcoming the overfitting challenge. The source code is available at \url{https://github.com/dazhangyu123/ACMIL}.

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