The Saethre-Chotzen syndrome is an autosomal, dominantly inherited craniosynostosis caused by mutations in the basic helix-loop-helix transcription factor gene TWIST1. This syndrome has hitherto not been associated with an increased risk of cancer. However, recent studies, using a murine breast tumor model, have shown that Twist may act as a key regulator of metastasis and that the gene is overexpressed in subsets of sporadic human breast cancers. Here, we report a novel association between the Saethre-Chotzen syndrome and breast cancer. In 15 Swedish Saethre-Chotzen families, 15 of 29 (52%) women carriers over the age of 25 had developed breast cancer. At least four patients developed breast cancer before 40 years of age, and five between 40 and 50 years of age. The observed cases with breast cancer (n = 15) are significantly higher than expected (n = 0.89), which gives a standardized incidence ratio (SIR) of 16.80 (95% CI 1.54-32.06). Our finding of a high frequency of breast cancer in women with the Saethre-Chotzen syndrome identifies breast cancer as an important and previously unrecognized symptom characteristic of this syndrome. The results strongly suggest that women carriers of this syndrome would benefit from genetic counseling and enrolment in surveillance programs including yearly mammography. Our results also indicate that the TWIST1 gene may be a novel breast cancer susceptibility gene. Additional studies are, however, necessary to reveal the mechanism by which TWIST1 may predispose to early onset breast cancer in Saethre-Chotzen patients.