1. The incidence of anencephaly in Singapore was determined from hospital records for 1953–56 inclusive. Sixty‐eight cases were found in 88,069 births, or 0.77 per thousand. 2. Analysis of data according to maternal community revealed great heterogeneity (Table 1), with frequencies ranging from 0.0 per thousand in the Hakka group of Chinese to 6.5 per thousand in Sikhs. 3. 48.4 % of anencephalics were males. This unusually high proportion may be connected with the low frequency of primiparity in Singapore and high average birth weight of affected infants, compared with records from elsewhere. Neither maternal age nor parity had a significant effect on the incidence. 4. The causes of anencephaly are discussed. It is suggested that properties of the local external environment (such as type of water supply) are of less importance than dietary and other customs peculiar to different communities, as well as genetical differences. 5. Several reasons are given why anencephaly, and other congenital malformations of the central nervous system, should be regarded as quasi‐continuous variables, due to a threshold effect. A comparison with the ‘curly‐tail’ mutant in the mouse supports this view and also emphasizes the close causal connexion between anencephaly and spina bifida. I am very grateful to Prof. L. S. Penrose, F.R.S., for arousing my interest in this subject and for invaluable advice. I should like to thank Prof. B. H. Sheares, Head of the University of Malaya's Unit at Kandang Kerbau Hospital, and Dr A. Arulanandam, Medical Superintendent, for arranging that I should have access to hospital records. I am greatly indebted to my wife for considerable help in the task of searching through these records. Copyright © 1959, Wiley Blackwell. All rights reserved