Phacomatosis pigmentokeratotica (PPK) represents a specific "twin nevus" syndrome in which a speckled lentiginous nevus (SLN) is associated with an organoid nevus with sebaceous differentiation. A boy with a large nevus sebaceus on the left face and upper part of the trunk, a giant segmental SLN extending from the abdomen to the feet bilaterally, and right hemihypertrophy developed an embryonal rhabdomyosarcoma of the right abdominal wall at age 6 months. A variety of musculoskeletal, neurologic, and ocular anomalies have been observed in patients with PPK, reflecting the individual manifestations of both SLN and Schimmelpenning syndromes. This report adds hemihypertrophy to the spectrum of extracutaneous manifestations of PPK and, to our knowledge, represents the first observation of a rhabdomyosarcoma arising in contiguity with an SLN in a patient with PPK. The development of a rhabdomyosarcoma in our patient likely reflects both increased propensity for growth (as evidenced by the hemihypertrophy) and the pluripotent nature of neural-crest derived cells within the field defect that underlies an SLN.