The purpose of this study was to investigate whether the relation between intelligence and season of birth, reported by some British and other workers, can be confirmed (1) in a sample of 708 normal Swedish children and (2) in a sample of 237 Swedish children below normal intelligence. In the normal sample, no significant difference in intelligence was obtained between groups of children born during different parts of the year. In the sample of backward children a trend was observed; the mean i.q. was highest in the group consisting of children born during the autumn.