Seasonal trends of some congenital CNS malformations in infants born in Sweden during the years 1965 to 1972 were investigated. The study is based on reports to the Swedish Register of Congenital Malformations, statistics from the National Board of Health, and records from Malmö General Hospital and the Hospital of Lund. The malformations analysed in this study are anencephaly, spina bifida aperta (with or without hydrocephalus) and isolated hydrocephalus. Variations in incidence of months of birth and last menstrual period (LMP) were tested statistically in three different ways: standard X2-test for heterogeneity between recorded numbers of infants each calender month, Edwards' method, and a squared sinus function, all with or without correction for variations in general monthly birth rates. Statistical significant seasonal variations were found for anencephaly with an LMP maximum in March and for spina bifida aperta with a maximum in July. Factors which can explain such seasonality are briefly discussed.