The influence of maternal position on the spread of local anesthetics in low concentration has not been well examined during epidural analgesia for labor. This study was designed to investigate the differences in sensory block, pain relief and incidence of supine hypotensive syndrome between parturients in the left lateral position and in a modified supine position. Sixty-seven parturients were randomly assigned to lie either in the left lateral position (n = 34) or in a modified supine position (n = 33), and received 0.125% bupivacaine 10 mL with epinephrine 1:800000 and sufentanil 7.5 microg. At 20 min parturients in the modified supine position turned to the left lateral position and a second investigator, unaware of the initial position, measured the extent of the sensory block at 20 and 30 min and just before a second epidural injection was requested. More dermatomes were blocked on the dependent side when the dose was injected in the left lateral position (at 20 and 30 min: P < 0.05; before the second epidural injection: P < 0.0005). In the modified supine position the incidence of bilaterally blocked dermatomes T10-L1 was greater at 20 and 30 min (P < 0.05) and the pain on a visual analogue scale was better at 30 min (P < 0.05). Three parturients in the modified supine position had signs and symptoms of supine hypotensive syndrome. We conclude that injecting in the modified supine position results in a more equal spread of local anesthetic and better pain relief.