Since January 1989 we have ascertained patients with neurofibromatosis type 1 (NF1) as part of our genetic register in the North West of England. This register has now identified 453 affected cases from 235 families. The first 94 individuals were specifically examined for features of the Noonan phenotype. This was present in 12/94 sequentially identified individuals with NF1 including six individuals from three families. However, three cases occurred in a further family, where Noonan syndrome appeared to segregate separately from NF1. We have provided evidence for the chance association of Noonan syndrome and NF1 and that the Noonan phenotype occurs as a feature in some NF1 families. However, there is now little evidence of a separate NF1/Noonan syndrome entity or of NF1 features occurring in classical Noonan syndrome.