Atypical stromal cells (ASCs) may be detected in endometrial polyps and, more rarely, in normal endometrium. Owing to their worrisome cytological features, these cells may represent a potential diagnostic pitfall and are often misinterpreted as malignant, particularly in biopsy samples. We report on ASCs in proliferative phase endometrium of a woman who underwent biopsy for vaginal bleeding. Morphological and immunohistochemical features are discussed in detail. The differential diagnosis to endometrial malignant tumors containing atypical mesenchymal cells is provided.