Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder caused by mutation in the NF1 tumor-suppressor gene, and may sometimes manifest in a mosaic form. “Segmental NF1” is generally assumed to be the result of somatic mosaicism for a NF1 mutation, and patients with mosaic NF1 have typical features of NF1 limited to specific body segments. The clinical features of 58 patients (42 females and 16 males; aged 1–69 years; mean age, 23.4 years) with mosaic NF1 seen at the Jikei University Hospital during 2004–2007 and at the Jikei University Daisan Hospital during 2007–2011, were retrospectively studied. Somatic or gonosomal mosaicism was not investigated. Patients were classified into four groups: (i) pigmentary changes (café-au-lait spots and freckling) only (n = 32); (ii) neurofibromas only (n = 5); (iii) neurofibromas and pigmentary changes (n = 13); and (iv) solitary plexiform neurofibromas (n = 8). The area of involvement was variable. The majority of patients were asymptomatic, except patients with plexiform neurofibromas who presented most commonly with pain or tenderness. Lisch nodules were rarely seen. Only four of our 58 patients (6.9%) had specific NF1 complications, including language delay (n = 1) and bone deformity (n = 3). Two patients were ascertained through their children with generalized NF1. Patients with mosaic NF1 are at low risk of developing disease-associated complications, except patients with plexiform neurofibromas. However, they need to be aware of the small risk of having a child with generalized NF1. [ABSTRACT FROM AUTHOR]