BACKGROUND: Conventional intermittent "Top-up" injections (CIT) of fentanyl added to bupivacaine reduces the total amount of local anesthetic required, resulting in less motor blockade, reducing incidence of instrumental deliveries, and improving the quality of analgesia during labor. The purpose of this study was to compare the analgesic efficacy of two different solutions and side effects of them. METHODS: Fifty healthy parturients were divided into two groups: intermittent epidural injections of 0.25% bupivacaine with 0.0005% fentanyl (Group I) and 0.125% bupivacaine with 0.0005% fentanyl (5 microgram/ml) (Group II). We injected 5 ml of each solution to block T10 sensory level of the request of the parturients. Assessments included interval of epidural injection, duration of first and second stage of labor, total dose of bupivacaine, maternal blood pressure, satisfaction, fetal heart rate, and Apgar scores. RESULTS: The interval of epidural injection was 75.7+/-50.6 min in Group I, compare with 57.8+/-27.5 min in Group II (p<0.05). The total dose of bupivacaine was 45.5+/-15.3 mg in Group I, compared with 23.5+/-10.4 mg in Group II (p<0.05). There was no significant difference between the two groups in terms of duration of labor, maternal blood pressure, maternal satisfaction, fetal heart abnormality, or Apgar scores of neonates. No serious side effects were encountered in either group, although two patients in Group I experienced urinary retention. Only one patient in each group experienced nausea and vomiting. CONCLUSIONS: The injection of 0.125% bupivacaine plus fentanyl provides good analgesia during labor. We concluded that lower concentration of bupivacaine was better in consideration of adverse effects of it on fetus and mother.