A 53-year-old black man was diagnosed with poorly differentiated adenocarcinoma of the lung and treated initially with 4 cycles of paclitaxel in combination with carboplatin and external-beam radiation therapy with a good clinicoradiologic response. The patient tolerated the chemotherapy well and did not develop any skin or nail changes during that period of time. His lung cancer recurred 10 months later, when he was found to have bone metastases. Second-line chemotherapy with pemetrexed 500 mg/m2 intravenously every 3 weeks was commenced. A week prior, the patient was started on folic acid 1 mg orally daily and given an injection of vitamin B12 1000 μg intramuscularly that was continued every 3 cycles thereafter. Dexamethasone 4 mg orally twice daily was given around the time of chemotherapy administration to prevent the dermatitis associated with the drug. The patient denied taking other drugs. Two months into his second-line chemotherapy, he developed multiple, concomitant, transverse and longitudinal black lines in all of his fingernails and toenails. After an interval of 3 months, he presented a complex pattern of nail hyperpigmentation, from combined dense horizontal and longitudinal streaks in some nails to diffuse black discoloration in others (Figure). Other associated changes included koilonychia, dystrophy, and friability of nail plates. Along with normal results of a hepatorenal panel and normal serum vitamin B12 and folate levels, no metabolic or endocrinologic alterations were present to explain the nail pigmentation and dystrophic changes. Results of his mycologic examination and cultures came back negative. When questioned, he denied taking any other drugs including other alternative medicine approaches or vitamin supplements, particularly retinoids, well known for causing severe nail dystrophy.