BACKGROUND: With preventive aortic grafting decreasing the incidence of type A dissections in Marfan syndrome (MFS), most dissections are now type B, for which risk factors remain largely uncertain. OBJECTIVES: We explored the determinants of type B dissection risk in a large, single-center MFS registry. METHODS: Demographic and anthropometric features, cardiovascular disease, and surgical history were compared in patients with MFS with and without type B dissection. RESULTS: Of 336 patients with MFS, 47 (14%) experienced a type B dissection (vs type A in 9%). Patients with type B dissection were more likely to have undergone elective aortic root replacement (ARR) (79 vs 46%; P < 0.001). Of the patients, 55% had type B dissection a mean of 13.3 years after ARR, whereas 45% experienced type B dissection before or in the absence of ARR; 41 patients (87%) were aware of their MFS diagnosis before type B dissection. Among those with predissection imaging, the descending aorta was normal or minimally dilated (<4.0 cm) in 88%. In multivariable analyses, patients with type B dissection were more likely to have undergone ARR and independent mitral valve surgery, to have had a type II dissection, and to have lived longer. CONCLUSIONS: In our contemporary cohort, type B dissections are more common than type A dissections and occur at traditional nonsurgical thresholds. The associations of type B dissection with ARR, independent mitral valve surgery, and type II dissection suggest a more severe phenotype in the setting of prolonged life expectancy.