Abstract Objective Assess patients submitted to elective cesarean section under spinal anesthesia, and the efficacy of different doses of fentanyl associated with bupivacaine. Methods The study included 124 pregnant women randomly distributed into 4 groups (n = 31) according to different doses of fentanyl (15 µg, 10 µg, 7.5 µg), Groups I, II, and III, respectively, and control group IV, associated with 0.5% hyperbaric bupivacaine (10 mg). An epidural catheter was inserted in case epidural top-up was required. We assessed the anesthetic blockage characteristics, negative maternal and neonatal outcomes, and maternal side effects. Statistical analysis was performed using Kruskal-Wallis, Fisher’s exact and chi-square tests. The level of significance was 5% (p< 0.05). Results The quality of analgesia, time for the first complaint of pain and motor block recovery time were significantly better for groups that received fentanyl in comparison to controls (p< 0.001). None of the groups had negative maternal-fetal outcomes. Nausea was significantly more frequent in patients in Groups II (10 µg) and III (7.5 µg) when compared to Groups I (15 µg) and IV (no fentanyl). Vomiting was more frequent in Group III than in Group I (p= 0.006). The incidence of pruritus was significantly higher in the groups receiving fentanyl (p= 0.012). Conclusions Among the solutions studied, the spinal anesthesia technique using 15 µg of fentanyl associated with 10 mg of hyperbaric bupivacaine provided satisfactory analgesia and very low incidence of adverse effects for patients submitted to cesarean section. Trial Registration Number UTN U1111-1199-0285. REBEC RBR-5XWT6T.