Can segmental neurofibromatosis be considered a rare disease?

'True' segmental neurofibromatosis (SNF) is characterized by the unilateral, segmental appearance of neurofibromas and/or 'cafe au lait' spots in the absence of genetic transmission. SNF is apparently rare: only 41 cases have been described since the first report by Crowe in 1956. However, the incidence of SNF is probably higher because more and more patients have been described in the last few years and several cases have been observed by single trained authors in a short time. In 1988 and 1989 we had described two patients affected by 'true' SNF. Now we report three additional patients observed in the last two years. 'True' SNF is considered to be a consequence of a post-zygotic (hence non-transmissible) mutational event but this hypothesis is not suitable for cases with segmental lesions matching deep systemic involvement, bilateral distribution of cutaneous lesions or genetic transmission. Additional reports, with careful investigations of both systemic involvement in SNF patients and apparently unaffected relatives and with long follow-ups, will help clarify the genetics and the pathogenests of the disease.