Type 1 Neurofibromatosis, NF1, is a common genetic disorder with variable clinical manifestations. Although NF1 often is only of cosmetic concern, serious and even lethal complications may occur. It is not possible to predict which symptoms will develop in any affected individual. The NNFF International Database is a multicentre collaborative system for collecting information about this condition. At the time of this analysis, complete clinical information was available on 1,479 probands and 249 of their affected relatives with NF1. On average, the age at diagnosis of NF1 was 8 years younger in the probands than in the affected relatives (P<.01). Many of the manifestations of NF1 were more frequent in the probands than in their affected relatives. The age-specific prevalence of most manifestations of NF1 increases with age. Despite biases inherent in a convenience sample from specialist clinics, the frequencies of many of the serious manifestations of NF1 are similar to those of two smaller population-based studies. The frequencies in this study are likely representative of patients seen at specialized clinics.