Outcome of pregnancy after amniocentesis was studied in a randomised controlled trial of 4606 women, age-range 25-34 years, without known risk of genetic disease. Spontaneous abortion rate was 1.7% in the study group after amniocentesis and 0.7% in the control group after ultrasound (relative risk 2.3). In the study group, increased levels of maternal serum alpha-fetoprotein before amniocentesis, perforation of the placenta during amniocentesis, and withdrawal of discoloured amniotic fluid were associated with an increased risk of spontaneous abortion. In the first six weeks after amniocentesis/ultrasound scan, amniotic fluid leakage occurred more often in the study group but there was no difference in the rate of vaginal bleeding. Frequency of postural malformations in the infants in the two groups was the same. In the study group, respiratory distress syndrome was diagnosed more often (relative risk 2.1) and more babies were treated for pneumonia (relative risk 2.5).