In the 2 years between 1993 and 1995, we assayed alpha-fetoprotein (AFP) in 48 412 amniotic fluids (AFs). One thousand and eighty-six (2โข2 per cent) measured _2โข0 MOM and these were subdivided into three groups; mildly (2โข0-4โข9 MOM), moderately (5โข0-9โข9 MOM), and very elevated (_10โข0 MOM). Abnormalities occurred in 25 per cent of the mildly elevated compared with 88 per cent of the moderately and 98 per cent of the very elevated cases. Forty-five per cent of the neural tube defects (NTDs) had AF-AFPs in the 5โข0-9โข9 and 36 per cent in the _10โข0 MOM range. After a positive acetylcholinesterase (AChE) test, both the moderately and the very elevated groups had abnormalities in _95 per cent of cases, compared with 85 per cent in the mildly elevated group. After a negative AChE test, abnormalities occurred in 79, 52 and 18 per cent in the very elevated, moderately, and mildly elevated groups, respectively. Excluding chromosome abnormalities, an abnormal twin, and bloody samples, the risk of a fetal abnormality after a normal ultrasound was less than 1 per cent if the AChE was positive in both the moderately and the very elevated groups. If the AChE was negative, the risk was 18 per cent for the moderately and 55 per cent for the very elevated groups, of which congenital nephrosis accounted for 75 per cent; AFP levels usually increased in a later AF sample. Repeat amniocentesis may be offered to women with AF-AFPs _5โข0 MOM if no abnormality is seen with high-resolution ultrasound. 1997 by John Wiley & Sons, Ltd.