A retrospective study of birth and death certificates for 6 consecutive years 1961-1966 in a single state revealed 367 anencephalic births, an incidence of 0.826 per 1,000. A steadily rising incidence from 0.58 per thousand 1961 to 1.01 per thousand 1966 was observed. Incidence of this malformation in the Caucasian was found to exceed by almost sevenfold that in the Negro population. Seasonal and geographic factors were shown to have no direct effect on incidence of the defect. Previous reports suggesting an excess of females (64 per cent of the present series), premature delivery (58 per cent of this series), and fetal growth retardation (57 per cent of this series) were confirmed. An excessive perinatal mortality of 112.9 per thousand was recorded in the 204 multiparous mothers in this series, confirming the concept that birth of an anencephalic fetus may predict or accompany a disastrous obstetric career. © 1969.