Spinal anesthesia in day-care surgery is still controversial because of the possibility of postdural puncture headache (PDPH). The use of the Sprotte needle with a conical tip that spreads the dural fibers may reduce the incidence of PDPH. The aim of this study was to compare the 22-gauge and 25-gauge Sprotte needles with respect to PDPH and postoperative complaints in out-patients undergoing arthroscopy. The rate of spinal anesthesia failure and the feasibility of unilateral spinal anesthesia when using a low dose of anesthesia was also verified. For this randomized, prospective study, 234 patients undergoing elective arthroscopy were chosen. Patients were allocated randomly to have spinal anesthesia with either a 22-gauge or 25-gauge Sprotte needle. Dural puncture was performed with the patient in a lateral flexed decubitus position. After the injection of anesthetic solution (0.5-1.2 mL of 1% bupivacaine in 8% glucose) patients remained in the lateral decubitus position for 30 min. The time to regression of analgesia, time of micturition, and all postoperative complaints were recorded. The most frequent side effect was backache (10.2%) with a more frequent incidence in the group using the 22-gauge needle (14.5% and 5.9%, respectively). PDPH was recorded in only one patient (0.8%) in the group using the 22-gauge needle. The failure rate was 0.8%. Unilateral anesthesia was achieved in 88% of 213 patients. Our data indicate that the use of spinal block is a suitable technique in the ambulatory setting, with a low rate of unplanned hospital admission.(ABSTRACT TRUNCATED AT 250 WORDS)