BACKGROUND: Hypotension is the most frequent side effect of spinal anesthesia. Unilateral spinal anesthesia may be advantageous because it reduce the extent of spinal block. The aim of this study was to compare the incidence of hypotension and the difference of heart rate between unilateral and conventional bilateral spinal anesthesia. METHODS: Seventy patients were randomly allocated into two groups, which both received 2.0 ml (10 mg) of 0.5% hyperbaric bupivacaine. In group 1, local anesthetic was injected for two minutes with the needle orifice turned toward the dependent side; the lateral position was maintained for 20 minutes (unilateral, n = 35). In group 2, local anesthetic was injected through a cranially directed needle orifice, then patients were immediately turned supine (conventional, n = 35). Observers recorded noninvasive hemodynamic variables, as well as loss of cold and touch sensation and motor block on both side. RESULTS: In the unilateral group, 16 patients (45.7%) showed a unilateral loss of cold sensation and 23 patients (65.7%) had no motor block on the nondependent side for the duration of the study, whereas all conventional patients had bilateral distribution of spinal block (P < 0.001). The incidence of hypotension was higher in the conventional (22.9%) than unilateral group (5.7%) (P < 0.01). Considering the mean changes from baseline values of arterial blood pressure, patients of the conventional group showed greater decreases in systolic arterial blood pressure at 40, 50, 60, and 70 min (P < 0.05). Maximum percentage changes from baseline values of systolic arterial blood pressure and heart rate were greater in the conventional group (-17.8 +/- 9.8% and -19.7 +/- 10.4%) than in unilateral group (-14.6 +/- 5.5% and -16.9 +/- 11.2%). CONCLUSIONS: Unilateral spinal anesthesia reduces the incidence of hypotension during spinal anesthesia.