The goal was to determine the relative effects of underlying genetic factors and current management strategies on neurodevelopmental disabilities among one-year old survivors of palliation for hypoplastic left heart syndrome. Children who underwent staged reconstruction for hypoplastic left heart syndrome and variants were assessed at 1 year of age by using a neuromuscular examination and the Bayley Scales of Infant Development II, which provide the Mental Development Index and the Psychomotor Development Index. The effects of perioperative, operative, and genetic variables on developmental scores were evaluated. The median birth weight was 3.3 kg (range: 2.1-4.5 kg). Eight-three patients (94%) underwent multiple operations with cardiopulmonary bypass during the first year of life (median: 2 operations). Seven patients (8%) required extracorporeal membrane oxygenation. Twenty-five patients (28%) had a confirmed or suspected genetic syndrome. At 1 year of age, the neuromuscular examination results were abnormal or suspect for 57 patients (65%). The median Mental Development Index score was 90, and 10 patients (11%) had scores of <70 (2 SDs below the general population mean). The median Psychomotor Development Index score was 73, and 42 patients (48%) had scores of <70. In multivariate analyses, younger gestational age, the presence of a genetic syndrome, and the need for preoperative intubation had significant negative effects on neurodevelopmental outcomes. No association was found with operative factors, including duration of deep hypothermic circulatory arrest. At 1 year of age, there was a significant incidence of neurodevelopmental disabilities in children with hypoplastic left heart syndrome and variants; motor scores were particularly concerning. Many children had suspected or confirmed genetic syndromes, which negatively affected neurodevelopmental outcomes. Surgical variables did not affect neurologic outcomes.