It is unclear whether the volume or concentration of local anesthetic influences its spread and quality of caudal analgesia when the total drug dose is fixed.We performed this study in a prospective, randomized, observer-blind manner. Children aged 1-5 yr received a constant dose of 2.25 mg/kg of ropivacaine prepared as either 1.0 mL/kg of 0.225% (low volume/high concentration [LVHC], n = 37) or 1.5 mL/kg of 0.15% solution (high volume/low concentration [HVLC], n = 36). Both solutions contained radiopaque dye.The median spread levels with ranges in the HVLC group (confirmed by fluoroscopic examination) were significantly higher (T6, T3-11) than in the LVHC group (T11, T8-L2). There were no significant differences in recovery times, postoperative pain scores, or side effects between the two groups. After discharge, fewer children in the HVLC group required rescue oral acetaminophen compared with the LVHC group (50.0% vs 75.7%). First oral acetaminophen time was found to be significantly longer with HVLC patients than LVHC patients (363.0 min vs 554.5 min).We confirmed (with fluoroscopy) that a caudal block with 1 mL/kg ropivacaine spreads to T11 and to T6 with 1.5 mL/kg. If the total dose is fixed, caudal analgesia with a larger volume of diluted ropivacaine (0.15%) provides better quality and longer duration after discharge than a smaller volume of more concentrated ropivacaine (0.225%) in children undergoing day-case orchiopexy. The spread level of ropivacaine correlated significantly with the first oral acetaminophen time after discharge.