<b><i>Background:</i></b> Despite their sometimes deceivingly bland appearance, some small-cell lymphomas are very aggressive and the prognosis of patients depends on a prompt diagnosis that allows the initiation of appropriate therapy. <b><i>Summary:</i></b> The present review discusses the salient cytological features of the most common aggressive small-cell lymphomas and then proceeds to analyze their main diagnostic criteria, including the usage of ancillary techniques. <b><i>Key Messages:</i></b> Lymph node fine-needle aspiration cytology is a fast, safe, cheap, minimally invasive, and accurate procedure that can be used for a prompt and accurate diagnosis of lymphomas.