The R2726W mutation in the fibrillin 1 (FBN1, Marfan syndrome) gene segregates with isolated skeletal features of Marfan syndrome and/or high stature. Here we report a family in which two out of four individuals, an 18-year-old son and his mother, a 41-year-old woman, had the R2726W mutation of FBN1. Both family members carrying the mutation were of average height. The son had a Marfan-like phenotype, but his mother did not. The FBN1 R2776W mutation, which is associated with skeletal features of Marfan syndrome, appears incompletely penetrant. Consequently, genetic counselling in the presence of this mutation is difficult.