The relationship between the arterial oxygen saturation level and performance on a centrally mediated task was determined in patients with congenital heart disease. The critical flicker frequency (CFF) of 212 acyanotic and 66 cyanotic patients with congenital heart disease, with arterial oxygen saturation levels below 90%, was determined by a standardized technique. The CFFs of the cyanotic patients as a group were significantly lower than those of the acyanotic patients. Within the cyanotic group, the patients with the lower arterial oxygen saturation level had the lower CFFs. These data support the hypothesis that the hypoxemia associated with cyanotic congenital heart disease does impair central nervous system functioning.