Maternal serum alpha-fetoprotein (AFP) levels were determined before and after genetic amniocentesis (n=283) or ultrasound scan (n=268) in a group of women participating in a randomized trial of genetic amniocentesis. Increases in AFP levels were seen significantly more often after amniocentesis than after ultrasound scan (P<0·001) and 48 women in the amniocentesis group had increases in AFP (ΔAFP) that were larger than the maximum ΔAFP (±25 μg/1) seen in the ultrasound group. If a ΔAFP of <25 μg/1 represents a feto-maternal haemorrhage (FMH) attributable to amniocentesis, the rate of amniocentesis-induced FMH was 17%. Such FMH was seen significantly more often after transplacental amniocentesis or after amniocentesis performed by less experienced operators. No association was detected between birthweight and FMH attributable to amniocentesis.