BACKGROUND: Inguinal hernia repair, hydrocelectomy, and orchidopexy are commonly performed surgical procedures in children. Postoperative pain control is usually provided with a single-shot caudal block. Blockade of the ilioinguinal nerve may lead to additional analgesia. The aim of this double-blind, randomized controlled trial was to evaluate the efficacy of an adjuvant blockade of the ilioinguinal nerve using ultrasound (US) guidance at the end of the procedure with local anesthetic vs normal saline and to explore the potential for prolongation of analgesia with decreased need for postoperative pain medication. METHODS: Fifty children ages 1-6 years scheduled for unilateral inguinal hernia repair, hydrocelectomy, orchidopexy, or orchiectomy were prospectively randomized into one of two groups: Group S that received an US-guided ilioinguinal nerve block with 0.1 ml x kg(-1) of preservative-free normal saline and Group B that received an US-guided nerve block with 0.1 ml x kg(-1) of 0.25% bupivacaine with 1 : 200,000 epinephrine at the conclusion of the surgery. After induction of anesthesia but prior to surgical incision, all patients received caudal anesthesia with 0.7 ml x kg(-1) of 0.125% bupivacaine with 1 : 200,000 epinephrine. Patients were observed by a blinded observer for (i) pain scores using the Children and Infants Postoperative Pain Scale, (ii) need for rescue medication in the PACU, (iii) need for oral pain medications given by the parents at home. RESULTS: Forty-eight patients, consisting of 46 males and two females, with a mean age of 3.98 (SD +/- 1.88) were enrolled in the study. Two patients were excluded from the study because of study protocol violation and/or alteration in surgical procedure. The average pain scores reported for the entire duration spent in the recovery room for the caudal and caudal/ilioinguinal block groups were 1.92 (SD +/- 1.59) and 1.18 (SD +/- 1.31), respectively. The average pain score difference was 0.72 (SD +/- 0.58) and was statistically significant (P < 0.05). In addition, when examined by procedure type, it was found that the difference in the average pain scores between the caudal and caudal/ilioinguinal block groups was statistically significant for the inguinal hernia repair patients (P < 0.05) but not for the other groin surgery patients (P = 0.13). For all groin surgery patients, six of the 23 patients in the caudal group and eight of the 25 patients in the caudal/ilioinguinal block group required pain rescue medications throughout their entire hospital stay or at home (P = 0.76). Overall, the caudal group received an average of 0.54 (SD +/- 1.14) pain rescue medication doses, while the caudal/ilioinguinal block group received an average of 0.77 (SD +/- 1.70) pain rescue medication doses; this was, however, not statistically significant (P = 0.58). CONCLUSIONS: The addition of an US-guided ilioinguinal nerve block to a single-shot caudal block decreases the severity of pain experienced by pediatric groin surgery patients. The decrease in pain scores were particularly pronounced in inguinal hernia repair patients.