Melanonychia striata longitudinalis is due to a focal increase in number and/or activity of melanocytes in the nail matrix, resulting in the continuous production of large amounts of melanin which are transferred to the nail cells and grow out with the nail plate. Histologically, either benign melanocytic hyperplasia, lentigo simplex, nevocellular nevus, atypical melanocytic hyperplasia or acral lentiginous melanoma may be found. Longitudinal pigmented streaks in the nails of Caucasians tend to be malignant rather than benign. Thus a biopsy for proper histological diagnosis is strongly advised. Melanonychia striata may also occur as a symptom of rare syndromes and diseases or be due to therapy. Differentiating this condition from subungual hematoma and infections with chromogenic bacteria and fungi may be difficult.