Sarcomatoid mesotheliomas can be challenging to diagnose on small biopsy specimens, where limited material may preclude definitive assessment of invasion, and lesional cells can have relatively bland cytology with no mesothelial marker expression. We report a case of a patient who presented with a pleural effusion and had subsequent pleural biopsy that showed a bland, uniform spindle cell proliferation in a mildly myxoid background. There was little, if any, collagen and no chest wall, soft tissue, or fat, and mesothelial markers were negative. The cells were positive for pancytokeratin and GATA3 by immunohistochemistry, and in situ hybridization showed a “negative” result for homozygous loss of CDKN2A; however, there was partial (heterozygous) loss of 1 allele. A diagnosis of atypical spindle cell proliferation was made based on these findings. Several months later, the patient had a repeat pleural biopsy that showed spindled cells with more pleomorphism, areas of invasion into the chest wall, and the same partial loss of CDKN2A, consistent with a sarcomatoid mesothelioma. This case underscores the challenges present on small biopsy specimens and the fact that sarcomatoid mesotheliomas can be relatively bland-appearing with focal pleomorphism and that heterozygous loss of CDKN2A should be considered a positive result indicative of a neoplastic process.