Segmental neurofibromatosis type 1 (NF1) is an under-recognized form of NF1 caused by postzygotic somatic loss-of-function NF1 gene mutations that affect a subset of cells in the body.1 This is in contrast to classic or generalized NF1, in which a germline NF1 gene mutation affects all diploid cells in the body. In the segmental NF1 variant, individuals typically exhibit clinical features characteristic of generalized NF1, such as café-au-lait macules (CALMs), skinfold freckling, and neurofibromas, restricted to one segment of the body. For this reason, establishing the diagnosis can be challenging because the underlying NF1 gene mutation is often not detected in the blood. Underscoring the challenges of caring for individuals with this variant of NF1, we describe a woman with segmental NF1 referred to us at 22 years of age for evaluation. © 2018 American Academy of Neurology.