The uterine circulation in 38 patients with invasive mole or choriocarcinoma were investigated using Doppler ultrasound. Twenty-six non-pregnant and 23 pregnant volunteers were assessed in a similar fashion to act as control groups. The pulsatility indices were significantly lower in the patients with trophoblastic disease when compared with the non-pregnant and pregnant groups (Student's t-test; P less than 0.001 and P less than 0.02 respectively). The uterine circulation in persistent trophoblastic diseases shows a characteristic Doppler profile suggesting the presence of large low resistance blood vessels. This may be useful in the initial assessment of these tumours before cytotoxic chemotherapy.