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Although mutations in cardiac sodium and potassium channel genes are associated with congenital long QT syndrome (LQTS), a 'modifier' role of the sympathetic nervous system was proposed to explain the distinct severity of the disease. We evaluated cardiac sympathetic innervation using [sup.11]C]hydroxyephedrine ([[sup.11]C]HED) and positron emission tomography (PET) in genotyped LQTS patients. [H.sub.2][sup.15]O and [[sup.11]C]HED PET studies were performed in 11 patients (5 symptomatic) and 8 controls. Perfusion and [[sup.11]C]HED images were depicted as 36-sector polar maps. Sectorial values of perfusion ([H.sub.2][O.sub.%]), absolute (HE[D.sub.Ret]) and relative retention (HE[D.sub.%Ret]) of [[sup.11]C]HED, and the ratio of HE[D.sub.%Ret] to [H.sub.2][O.sub.]% (HE[D.sub.%Ret]/[H.sub.2][O.sub.%]) were calculated. Normal databases were obtained from controls. Sectorial values below 2SD database values were defined as 'outside sectors.' Controls and patients showed similar sectorial perfusion. Sectorial HE[D.sub.Ret] did not differ between groups, but means of HE[D.sub.%Ret] were lower in three sectors for patients (P < 0.05). Three sectors from 3 controls had HE[D.sub.%Ret] below 2SD, whereas 36 sectors in 9 patients were outside sectors (P < 0.01). In patients, average HE[D.sub.%Ret]/[H.sub.2][.sub.O%] was lower in 9 sectors (P < 0.05 vs. controls); 2 outside sectors were found in controls, but 43 outside sectors were found in patients (P < 0.01), 77% of them in the 5 symptomatic patients. Heterogeneous [[sup.11]C]HED retention was localized in the septal, anterior, and lateral walls. Most LQTS patients showed a localized and decreased pattern of [[sup.11]1C] HED retention. The larger number of heterogeneous sectors in symptomatic patients suggests that sympathetic function could play an amplifier role for severity of the disease. long QT syndrome; genetics; positron emission tomography; sympathetic nervous system; [[sup.11]C]hydroxyephedrine