We present monozygotic twins discordant for the autosomal dominant disorder neurofibromatosis type 1 (NF1). The affected twin was diagnosed with NF1 at age 12, based upon accepted clinical criteria for the disorder. Both twins were re-examined at ages 35 and 57, at which times the unaffected twin continued to show no clinical manifestations of NF1. Short tandem repeat marker (STR) genotyping at 10 loci on chromosome 17 and 10 additional loci dispersed across the genome revealed identical genotypes for the twins, confirming their monozygosity. The affected twin has three children, two of whom also have NF1, while the unaffected twin has two children, both unaffected. Using lymphoblastoid, fibroblast, and buccal cell samples collected from both twins and from other family members in three generations, we discovered a pathogenic nonsense mutation in exon 40 of the NF1 gene. This mutation was found in all cell samples from the affected twin and her affected daughter, and in lymphoblastoid and buccal cells but not fibroblasts from the unaffected twin. We also found a novel non-synonymous change in exon 16 of the NF1 gene that was transmitted from the unaffected mother to both twins and co-segregated with the pathogenic mutation in the ensuing generation. All cells from the twins were heterozygous for this apparent exon 16 polymorphism and for single nucleotide polymorphisms (SNPs) within 2.5 kb flanking the site of the exon 40 nonsense mutation. This suggests that the NF1 gene of the unaffected twin differed in the respective lymphoblastoid cells and fibroblasts only at the mutation site itself, making post-zygotic mutation leading to mosaicism the most likely mechanism of phenotypic discordance. Although the unaffected twin is a mosaic, the distribution of the mutant allele among different cells and tissues appears to be insufficient to cause overt clinical manifestations of NF1.