Background —Patients with a single ventricle have multiple risk factors for central nervous system injury, both before and after the Fontan procedure. Methods and Results —A geographically selected cohort was invited to undergo standardized testing, including age-appropriate measures of intelligence quotient (IQ) and achievement tests. Historical information was obtained by chart review and patient questionnaires. Of the 222 eligible patients, 133 (59.9%) participated. Median age at testing was 11.1 years (range, 3.7 to 41.0 years), 6.0 years (range, 1.6 to 19.6 years) after surgery. Mean full-scale IQ was 95.7±17.4 ( P <0.006 versus normal); 10 patients (7.8%) had full-scale IQ scores <70 ( P =0.001). After adjustment for socioeconomic status, lower IQ was associated with the use of circulatory arrest before the Fontan operation ( P =0.002), the anatomic diagnoses of hypoplastic left heart syndrome ( P <0.001) and “other complex” ( P =0.05), and prior placement of a pulmonary artery band ( P =0.04). Mean composite achievement score was 91.6±15.4 ( P <0.001 versus normal); 14 patients (10.8%) scored <70 ( P <0.001). After adjustment for socioeconomic status, independent risk factors for low achievement scores included the diagnoses of hypoplastic left heart syndrome ( P =0.004) and “other complex” ( P =0.003) or prior use of circulatory arrest ( P =0.03), as well as a reoperation with cardiopulmonary bypass within 30 days of the Fontan ( P =0.01). Conclusions —Most individual patients palliated with the Fontan procedure in the 1970s and 1980s have cognitive outcome and academic function within the normal range, but the performance of the cohort is lower than that of the general population.