Study objective - The aims were (1) to determine whether in Europe, 1980-86, geographical differences in total prevalence of neural tube defects persist; (2) to examine the stability of total prevalence rates over time; (3) to evaluate the impact of prenatal diagnosis in terms of frequency and timing of termination of pregnancy. Design - Prevalence rates of neural tube defects were determined from case registration data in 20 EUROCAT regional registers of congenital anomalies, 1980-86. The χ2 test for homogeneity in proportions was used to test whether differences in total prevalence rates were significant between regions or over time. Setting - Geographically defined populations were used in the Republic of Ireland, United Kingdom, Belgium, The Netherlands, Luxemburg, Denmark, France, Italy, Yugoslavia, and Malta. Patients - The patients were 3113 cases of anencephaly, spina bifida, encephalocele, and iniencephaly. Total cases (livebirths, stillbirths and induced abortions following prenatal diagnosis) were registered in 14 regions. Induced abortions were excluded from registration in six regions. Measurements and main results - Total prevalence rates (including livebirths, stillbirths and induced abortions) were 24 to 38 per 10000 in six areas of Ireland and United Kingdom. Average total prevalence rate in eight continental European areas was 11.5 per 10000. There was a secular decline in total prevalence in Dublin (Republic of Ireland) and Northern Ireland (United Kingdom) and a fluctuation in Glasgow, Liverpool, and South Glamorgan (United Kingdom). Total prevalence in continental Europe was stable over time. There was no signifcant geographical or secular variation in the spina bifida to anencephaly ratio (1.3). The ratio of encephalocele to other neural tube defects was lower in the British Isles (0.09) than in continental Europe (0.18). The impact of prenatal diagnosis and termination of pregnancy is increasing over time. Terminations were performed 1984-86 in at least 80% of total cases of anencephaly in 6/11 centres registering induced abortions, and in at least 40% of total cases of spina bifida in four centres. Serum α fetoprotein screening in British centres was associated with earlier prenatal diagnosis of spina bifida than ultrasound screening in other centres. Conclusions - Geographical and secular variation in total prevalence of neural tube defects persists in Europe 1980-86, independent of the practice of prenatal diagnosis. There is considerable regional variation in the impact of prenatal diagnosis in terms of frequency and timing of diagnosis and pregnancy termination linked to different policies and practices of prenatal screening.