AbstractNeurofibromatosis 1 (NF1) is an autosomal dominant disease that predisposes individuals to developing benign neurofibromas. Some features and consequences of NF1 appear to result from partial deficiency of neurofibromin (Nfn), the NF1 gene protein product, as a result of haploinsufficiency for the NF1 gene. Other features and consequences of NF1 appear to involve total deficiency of Nfn, which arises as a result of either loss of function of the second NF1 allele or excess degradation of Nfn produced by the second allele in a particular clone of cells. We used immunofluorescence to assess the presence of Nfn in putative Schwann cells (S100B+) and non‐Schwann cells (S100B−) in 36 NF1‐derived benign neurofibromas classified histologically as diffuse or encapsulated. The S100B+/Nfn− cell population made up only 18% ± 10% (mean ± standard deviation) of the neurofibroma cells in both the diffuse and encapsulated neurofibromas. The proportion of S100B+/Nfn+ cells was significantly higher and the proportion of S100B−/Nfn− cells was significantly lower in diffuse neurofibromas than in encapsulated neurofibromas. We isolated S100B+/Nfn+, S100B+/Nfn−, and S100B−/Nfn+ cells by laser microdissection and, using X‐chromosome inactivation profiles, assessed clonality for each cell type. We showed that, although some neurofibromas include a subpopulation of S100B+/Nfn− cells consistent with clonal expansion of a Schwann cell progenitor that has lost function of both NF1 alleles, other neurofibromas do not show evidence of monoclonal proliferation of Schwann cells. Our findings suggest that, although clonal loss of neurofibromin function is probably involved in the development of some NF1‐associated neurofibromas, other pathogenic processes also occur. © 2011 Wiley‐Liss, Inc.