The use of low-dose aspirin for the prevention and treatment of pre-eclampsia and intrauterine growth restriction has been studied extensively during the last 10 years. The large clinical trials do not support routine prophylaxis or therapeutic administration of low-dose aspirin in high-risk pregnant patients. We report on 87 Israeli women who were recruited for the CLASP study (a collaborative trial for the prevention and treatment of pre-eclampsia and intrauterine growth restriction). Of these women, 41 were treated with 60 mg aspirin daily, and 46 with placebo. The fetal circulation was evaluated by repeated Doppler flow waveform measurements of the umbilical artery and descending aorta from 20 to 34 weeks of gestation. The peak-systolic/ lowest-diastolic velocity ratio, pulsatility index and resistance index were calculated. No significant differences in Doppler measurements were observed for any of the indices between the aspirin and placebo groups. There were no statistically significant changes in the flow parameters during the course of pregnancy in any of the women studied, nor was there a correlation between waveform analysis and pregnancy outcome. Thus, based on the present results, it appears that low-dose aspirin does not improve fetal circulation parameters.