OBJECTIVE: To explore the onset and progression of cardiac involvement in juvenile neuronal ceroid lipofuscinosis (JNCL). METHODS: The study population comprised an unselected group of 29 children and adolescents with genetically verified JNCL. We focused on T-wave abnormalities on an EKG, cardiac hypertrophy, and left ventricular systolic function on echocardiography, and heart rates and heart rate variability (HRV) on 24-hour EKG recordings. The surviving patients were observed for 7½ years. The 24-hour EKG recording was repeated after 3 years. RESULTS: Abnormally deeply inverted T waves were present in one-third of the initial EKG recordings and were reported as early as 14 years of age. We found coherence between the presence of repolarization disturbances of the ventricular myocardium at the initial recordings and risk of death during the observation period. At increasing age, heart rate and HRV, expressed as the vagal index (number of adjacent RR intervals deviating more than 6%), were significantly reduced, suggesting an age-dependent bidirectional effect of JNCL on heart rate: one through decreasing parasympathetic activity on the heart and the other through a direct negative influence on sinus node automaticity. Coherence between bradycardia and arrhythmia and occurrence of sinus arrests and atrial flutter with increasing age indicated an age-dependent decrease in sinus node activity also. In the early 20s, a high frequency of ventricular hypertrophy occurred. CONCLUSIONS: Progressive cardiac involvement with repolarization disturbances, ventricular hypertrophy, and sinus node dysfunction occur in JNCL. We recommend that the attention on heart involvement in JNCL and other neuronal ceroid lipofuscinosis subtypes should be intensified.