The follow-up of patients with adult congenital heart disease (ACHD) at a specialized medical unit is necessary for the patients’ appropriate medical care. However, limited information is available about cardiovascular events among ACHD patients. Here we investigated the type and frequency of cardiovascular events in ACHD patients in relation to disease complexity. We retrospectively analyzed the cases of 535 patients (median age 35 years) referred to our ACHD center between 2014 and 2017. We divided the patients into 3 groups based on their disease complexity. To evaluate the relationship between disease complexity and cardiovascular events, we performed univariate and multivariate survival analyses. The Simple, moderate, and complex disease groups accounted for 62%, 19%, and 19% of the patients, respectively. Apart from events related to atrial septal defect (ASD) trans-catheter treatment, the frequency of cardiovascular events was dependent on the disease complexity (event-free survival rates at 3 years were 85%, 65%, and 58%, respectively). The hazard ratios were 4.0 and 5.1 in the moderate and complex groups, respectively. With the exception of scheduled transcatheter intervention, cardiovascular events were strongly related to the disease complexity of original heart disease. However, cardiovascular events were not rare even in the simple ACHD group.