Abstract Background Ventricular arrhythmias (VAs) are an important source of concern among athletes. Purpose To determine the prognostic role of non-invasive and invasive diagnostic assessments among athletes with complex VAs. Methods We included 215 athletes (83% male; 28 [19-43] years) with complex VAs in a prospective, multi-center cohort study. At baseline, each athlete underwent a prespecified comprehensive diagnostic workup, which included cardiac magnetic resonance (CMR) in 202 subjects, and electrophysiology study (EPS)/electroanatomical mapping (EAM) in 190. The primary study endpoint was the occurrence of sudden death or sustained VAs during long-term follow-up. Results During a median follow-up of 6.2 (4.4-8.1) years, 16 (7%) athletes had a primary outcome event, including one sudden cardiac death. In univariable Cox models, sustained VAs at presentation (p<0.001), uncommon VA morphology(p=0.004), syncope(p=0.005), abnormal ECG (p=0.003), VAs induction/persistence at exercise testing(p=0.021), late gadolinium enhancement (LGE;p=0.006), low-voltage areas (p=0.003) and late potentials (p<0.001) at EAM, and sustained VA induction by EPS (p<0.001) were all associated with increased risk of primary outcome events. The association of late potentials and EPS inducibility with primary outcome events was confirmed after controlling for all non-invasive predictors at univariable analysis, and the incorporation of findings of invasive tests allowed improved prediction of primary outcome events over clinical/non-invasive findings in isolation (log-likelihood ratio for nested models, p=0.043). Younger age (p=0.032), being a professional athlete (p=0.006), and absence of LGE (p=0.029) were independently associated with higher odds of uninterrupted competitive sports practice until last follow-up. Conclusions A comprehensive diagnostic assessment integrating clinical/ECG findings with CMR and EPS/EAM in selected cases may provide important prognostic information among athletes with complex VAs.