This study obtained data on 22 fetuses in whom reversal of diastolic flow in the middle cerebral artery (MCA) was seen. In 59% of cases there was normal heart function and in 28% there was isolated tricuspid valve regurgitation. The majority (73%) of cases presented with normal fetal anatomy and most (82%) had a normal amniotic fluid index and normal fetal growth. In all cases, Doppler results of the umbilical artery and vein were normal. Of the women, 65% were on no medication and 73% did not smoke during pregnancy. The reversal of diastolic flow in the MCA was seen temporarily in the majority of cases (for 2-30 min); however, in one case with rhesus factor disease and a rim of ascites, it was observed for a longer period (about 2 h) and on the following day, intrauterine demise was recorded. In most of the cases without structural malformations, the neonatal outcome was normal. We conclude that reversed diastolic flow in the MCA is a rare and usually transient event. In one of our cases prolonged reversed flow preceded intrauterine demise. Therefore, this may be an ominous sign and careful fetal surveillance should be undertaken when this observation is made. There are a few possible pathomechanisms of reversed diastolic flow in the MCA. In the majority of cases the cause of the observed phenomenon remains unknown, but an increased pressure in the right ventricle and possible tricuspid regurgitation should be considered.