The incidence of major congenital anomalies of the central nervous system was determined in a study of more than 233,000 births in Brooklyn, New York hospitals during the years 1968 to 1979. Anencephaly, myelomeningocele, and hydrocephalus occurred at frequencies of 47.1, 42.4, and 49.3 cases per 100,000 total births, respectively. During the 12 years of the study, the incidence of myelomeningocele decreased at a linear rate of approximately 4.6 cases per 100,000 births per year. A review of comparable reports on births in the northeastern region of the United States shows that a relatively consistent linear decline in myelomeningocele incidence has been proceeding at this rate for the past 50 years. It is concluded that minor local fluctuations and variability in data collection have obscured the steady decrease in occurrence. No significant declines were seen in the incidence rates of anencephaly or hydrocephalus, nor were uniform secular trends for these anomalies evident from previous reports. Epidemiologic differences between myelomeningocele and anencephaly are noted and their common etiology is questioned.