In a longitudinal study, 19 infants who displayed the setting-sun eye phenomenon were observed during the first year of life. Nine of the infants showed no signs of illness, eight had an evident increase in intracranial pressure requiring surgical relief, and two had transient signs of increased intracranial pressure which resolved spontaneously. The setting-sun phenomenon could be elicited both by alteration of the infant's position and by removal of light, and it also occurred spontaneously. The effectiveness of the eliciting mechanism depended on the age of the infant. The component parts of the phenomenon consist of downward rotation of the eyeballs and retraction of the upper eyelids, sometimes accompanied by raising of the brow. The phenomenon can be observed in healthy infants, and its value in early recongnition of increased intracranial pressure is limited. The response might indicate increased intracranial pressure if it can be elicited by alteration of position in infants older than four weeks of age or if there is a marked response to removal of light in infants younger than eight weeks or older than 20 weeks of age, especially if the response is combined with constant or intermittent strabismus or undulating eye-movements.