Segmental neurofibromatosis type 1 (SNF1), characterized by the regionally limited distribution of neurofibromatosis type 1 (NF1) features, has been attributed to mosaicism for an NF1 gene mutation. The occurrence of classical NF1 in the offspring of a parent with SNF1 suggests that cutaneous mosaicism may be accompanied by gonadal mosaicism. We studied a girl with generalized NF1, and her mother who has SNF1. A recurrent nonsense mutation in exon 31 (R1947X) of the NF1 gene was identified in the lymphocyte DNA of the affected child by denaturing high-performance liquid chromatography and PCR/direct sequencing. DNA sequence analysis failed, however, to identify the R1947X mutation in peripheral lymphocytes, and in keratinocytes and fibroblasts cultured from affected and unaffected skin in the mother. DNA fragments containing exon 31 were then cloned from each cell line and these clones were screened using allele-specific PCR. The R1947X mutation was identified in 29 of 146 clones derived from keratinocytes from the affected region and in 12 of 136 clones derived from fibroblasts from the affected region, but in no clones derived from clinically unaffected tissues. These findings confirm that gonosomal mosaicism can occur in SNF1, with consequent important implications for genetic counselling.