A large series of patients with cyanotic congenital heart disease and inadequate pulmonary blood flow who required a systemic to pulmonary artery anastomosis was reviewed, along with a small current series of patients unselected as to surgical intervention.Thirty-eight per cent of 190 patients in the first group had definite spells consisting of paroxysmal hyperpnea and increased cyanosis, frequently progressing to loss of consciousness. In the current group, 13 of 24 had spells. In the combined groups, there were three convulsions, eight cerebral vascular accidents, and six deaths due to the spell.Age of onset varied from 1 month to 2 years, with the peak incidence between 2 to 3 months of age. The time of day at which spells occurred was recorded in only 15 patients in the large series: 11 occurred in the morning and four in the evening. In the smaller series, the patients who had only occasional episodes had them exclusively in the morning, whereas the severely affected patients had attacks at almost any time.Precipitating factors, in order of over-all frequency, were crying, defecation, and feeding.The most surprising finding was the lack of correlation of hyperpneic spells with resting arterial desaturation, although all arterial saturations obtained during attacks were quite low. Two patients with observed, typical paroxysms had arterial saturations under sedation of 93 and 98 per cent, in contrast to a patient with a 44-per cent saturation who had had no spells.