This study evaluates the reliability and validity of the Toddler-Preschooler Postoperative Pain Scale (TPPPS), an observational scale developed to be a clinically useful measure of postoperative pain in children aged 1-5 years. The TPPPS consists of 7 items divided among 3 pain behavior categories: (1) Vocal pain expression; (2) Facial pain expression; and (3) Bodily pain expression. These items were derived from preliminary studies by the authors and from other observational studies of children's pain behavior. Seventy-four children between the ages of 12 and 64 months seen for inguinal hernia or hydrocele repair were the subjects of the study. Subjects were observed postoperatively for six 5-min intervals, commencing with their awakening from anesthesia, using the TPPPS. Two raters independently observed 28 of the children to assess inter-rater reliability. Validity was assessed by relating TPPPS scores to the timing and type of analgesics used, visual analog and numerical scale pain ratings made by parents and nurses, and perioperative vital signs. The TPPPS was found to possess satisfactory internal reliability (Cronbach's alpha = 0.88). Inter-rater reliability was good, with kappas for the pain behavior items ranging from 0.53 to 0.78. Preliminary evidence of the scale's validity is provided by the sensitivity of the scale to analgesic regimen, the convergence between TPPPS scores and nurse and parent ratings of postoperative pain, and the associations found between TPPPS scores and perioperative vital signs.