Using data from the Northern Congenital Abnormality Survey, we describe trends in the prevalence of neural tube defects (NTDs), maternal age-specific prevalence, outcome of pregnancy and the sensitivity of antenatal diagnosis of NTD-affected pregnancies in the Northern Region during 1984-96. This population-based sample consisted of 934 NTDs: there were 403 (43.1%) with anencephaly, 472 (50.5%) with spina bifida and 59 (6.3%) with encephalocele. The total prevalence at birth was 17.9 per 10,000 births and terminations (95% confidence interval [CI] = 16.7, 19.0) with a significant reduction over time (chi(2) for trend = 7.2; P = 0.007). The overall birth prevalence was 5.6 per 10,000 births [95% CI = 5.0, 6.3]; there was also a significant reduction in birth prevalence with time (chi(2) for trend = 68.3; P < 0.0001). Maternal age-specific prevalence rates decreased with increasing age. The proportion of NTD pregnancies terminated increased from 60.3% (325 cases) during 1984-90 to 78.6% (293 cases) during 1991-96, whereas the proportion of livebirths declined from 31.7% (171 cases) to 15.0% (56 cases) (P < 0.001). The sensitivity of antenatal diagnosis was consistently high for anencephaly (98%) and increased significantly for spina bifida from 60% during 1984-90 to 85% during 1991-96 (P < 0.05). Ascertainment of all cases of NTD in the Northern Region revealed a twofold reduction in birth prevalence between 1984-90 and 1991-96. This has resulted from improvements in the accuracy of antenatal detection of NTD-affected pregnancies with an increase in terminations of pregnancy.