OBJECTIVE: This study aimed to describe the features and identify the predictors of ascending aorta dilatation in patients with congenital bicuspid aortic valve (BAV). METHODS: In 280 adult patients with isolated BAV undergoing echocardiography, multivariate logistic regression models, including clinical and echocardiographic variables, were developed to predict dilatation (aortic ratio exceeding 1.1) at both ascending and root level. Factors predicting aneurysm with surgical indication were also investigated. Classification tree models were used to identify factors influencing the probability of having a small aorta, normal aortic dimensions, a dilated ascending aorta or a dilated root (aortic phenotypes). RESULTS: Aortic dilatation was present in 83.2% patients, prevailing at the mid-ascending tract in 83.7% of them. Surgical indication criteria were reached in 43.2% patients. A small aortic root was found in 16 patients (5.7%), thereafter excluded from multivariate models predicting dilatation. Age (maximal risk at 50-60 years: OR=13.7; reference category: <30 years) and severe aortic stenosis (OR=23.8) independently predicted mid-ascending dilatation (p<0.001). Male gender (OR=4.1, p=0.001), age >60 (OR=2.6, p=0.022) and severe regurgitation (OR=3.9, p=0.011) were determinants of root involvement, while stenosis (> or =moderate; OR=0.3, p<0.001) was a protective factor. Aortic stenosis (any degree, OR=2.4) and hypertension (OR=4.3) were the most significant predictors of mid-ascending aneurysm reaching surgical indication. Classification analysis showed that increasing age significantly increased the prevalence of ascending dilation phenotype, stenosis increased the prevalence of small aorta phenotype, and male gender of root dilation phenotype. Once excluding patients with small aortas from the analysis, a positive correlation was observed between degree of stenosis and mid-ascending size (p=0.016). CONCLUSIONS: BAV patients constitute an importantly heterogeneous population in terms of risk and features of aortic disease. The most common condition is an ectasia of the mid-ascending tract, with unaffected or mildly involved root. If further confirmed, this could suggest that surgical approach may spare the root in most BAV patients. Mid-ascending dilatation is proportional to stenosis severity, suggesting a post-stenotic causative mechanism. Root dilatation is rarer, mostly observed in younger men, and unrelated to the presence and severity of stenosis. The two different aortic dilatation phenotypes (mid-ascending and root) may be subtended by different pathogeneses.