A 30-year-old man presented to the hematology clinic with a 4-month history of white lines across his fingernails. Five months before presentation, he had received a diagnosis of primary mediastinal large B-cell lymphoma and began to receive systemic chemotherapy. The nail changes had first appeared midway through his six cycles of treatment. Physical examination was notable for six transverse white lines that were visible on the nails on both hands. A diagnosis of transverse leukonychia — also known as Mees’ lines — was made. Mees’ lines occur when there is abnormal keratinization of the distal nail matrix. In this case, each of the patient’s six chemotherapy cycles was thought to correspond to a band of leukonychia. Mees’ lines can be differentiated from Muehrcke’s lines — a type of apparent leukonychia triggered by vascular changes to the nail bed — because Mees’ lines do not blanch. Mees’ lines are also smooth, which differentiates them from Beau’s lines (transverse depressions in the nail bed owing to an arrest of nail matrix growth). The patient was reassured about the benign nature of the nail changes. At the 10-month follow-up, the nail changes had resolved.