A 70-year-old woman presented with an eight-month history of transverse changes in all fingernails and toenails. She had a history of breast cancer treated with mastectomy followed by six cycles of docetaxel-based chemotherapy with concurrent radiotherapy, after which she remained in remission. Transverse nail changes first appeared two months after the start of chemotherapy, the last cycle ended four months before the examination. Physical examination revealed six transverse grooves on all nail plates. Fungal cultures, potassium hydroxide tests, and microscopic examination of the nails ruled out both fungal and bacterial infection. The diagnosis was Beau's lines, which are palpable transverse indentations caused by the temporary arrest of nail matrix proliferation through the entire thickness of the nail plate. Six lines corresponded to six cycles of chemotherapy.