A 66-year-old Japanese man had skin lesions on the left side of his trunk and the left upper extremity for approximately 10 years. The skin lesions were asymptomatic but increased gradually, which brought the patient to our hospital. On the initial examination, we noticed 5 mm x 5 mm to 15 mm x 5 mm, round or oval, light to dark brown keratotic papules on the left side of chest and abdomen and the left upper extremity. The papules were aligned in an S-shaped line on the trunk and in a straight line on the upper extremity. Clinical and histopathological findings led to a diagnosis of seborrheic keratosis that followed Blaschko's lines. To our knowledge, no such a case has been reported previously. Our case supports the hypothesis that seborrheic keratosis can be associated with genetic mosaicism.