BACKGROUND Recalled St. Jude Medical Riata defibrillator leads are prone to insulation failures with externalized conductors (ECs). Longitudinal studies are needed to guide lead management. OBJECTIVE The purpose of this study was to describe the dynamic nature of EC and the association with electrical abnormalities and lead extraction outcomes. METHODS A nationwide cohort established in 2012 of 295 patients with recalled Riata leads with dwell time 5.1 +/- 1.1 years, 34 ECs, and 19 electrical abnormalities were followed until death, lead discontinuation with fluoroscopy, or a new 2013 screening with fluoroscopy and device interrogation. RESULTS Fluoroscopic follow-up of 239 patients with normal baseline fluoroscopy revealed incident overt EC in 8 leads and borderline EC in 2 leads after 1.1 +/- 0.2 years, with an incidence rate of 3.7 per 100 person-years (95% confidence interval Fluoroscopic follow-up in 27 patients with baseline EC showed an increase in EC length of 4 +/- 1 mm (P <.001) after 1.1 +/- 0.3 years. Electrical follow-up in 276 patients with normal baseline electrical function demonstrated 20 incident electrical abnormalities after 1.0 +/- 0.3 years, with an incidence rate of 7.1 per 100 person-years (95% confidence interval 4.6-11.0). This rate was significantly higher in leads with baseline EC, with an adjusted incidence rate ratio of 4.4 (95% confidence interval 1.7-11.5, P = .002). In 15 extractions, all leads were removed, with 2 major complications. CONCLUSION The development of EC is a dynamic process despite long lead dwell time. ECs are associated with a higher risk of electrical abnormalities. Therefore, lead replacement should be considered, especially in patients with a long life expectancy.