Background: The influence of gender and age on phenotypic features of individuals with Marfan syndrome has not been systematically examined in a large cohort of both children and adults. Methods: We evaluated 789 Marfan patients enrolled in the NHLBI GenTAC Registry (53% male; mean age 31 [range: 1-86 years]). Females aged≥15 and males aged≥16 years were considered adults based on average age of skeletal maturity. Results: Children (n=183) and adults (n=606) (see Table) were comparable in revised Ghent systemic score, ectopia lentis, and most phenotypic features, including prevalences of aortic root dilatation and mitral prolapse. However, adults were more likely to have spontaneous pneumothorax, scoliosis, dural ectasia and striae. Prophylactic aortic root replacement and mitral valve surgery were rare during childhood vs. adulthood (2 vs. 35% and 1 vs. 9%, respectively, both p<0.0001). There was only one childhood aortic dissection (type B in 15 year old male). Overall females were more likely than males to have arachnodactyly (54 vs. 40%), scoliosis (60 vs. 49%), and dural ectasia (16 vs. 10%), all p<0.01. Adult males were more likely to have aortic root dilatation (92 vs. 84%), aortic regurgitation (55 vs. 36%) and to have undergone prophylactic aortic root replacement (47 vs. 24%), all p<0.001. Prevalence of prior aortic dissection was higher in males than females (25 vs. 18%, p=0.06), 44% of which were type B. Mean age at dissection (35 vs. 38 years), age at prophylactic surgery (32 vs. 35 years), and need for mitral valve surgery (9.5 vs. 10.0%) were similar between men and women, respectively. Conclusions: Pulmonary, skeletal and aortic complications, but not other phenotypic features, increase with aging in Marfan syndrome. Aortic aneurysms and prophylactic aortic surgery are more common in men. In the era of current surgical and medical treatment, aortic dissection, commonly type B, occurs in an appreciable proportion of Marfan patients, especially among men.