BACKGROUND: Despite years of research and clinical experience with acute appendicitis, the rate of complications in the pediatric age group continues to be high. OBJECTIVE: To characterize the profile of the child with appendicitis complicated by perforation or intraabdominal abscess. METHODS: Between 1 January 1985 and 31 December 1997 in our department, 581 children under the age of 14 years were clinically diagnosed as suffering from "acute appendicitis." The final diagnoses were: while appendix in 28 cases (4.8%), acute non-complicated appendicitis in 472 (81%), and complicated appendicitis in 81 (13.9%), including 51 cases of free perforation (8.7%) and 30 cases of intraabdominal abscess (5.2%). We retrospectively reviewed the charts of all children with complicated appendicitis and those of 70 randomly selected children with non-complicated appendicitis, and compared patient age, gender, weight percentile, past medical history, and course of the illness. RESULTS: The children with complicated appendicitis were significantly younger (P = 4.8 x 10(-7)), they had higher oral and rectal temperatures (P = 7.9 x 10(-8)), higher platelet count (P = 0.0008) and lower hemoglobin level (P = 0.004). No difference was found in white blood count (P = 0.41). Total delay from symptom onset to surgery was 33 hours (SD 23) in the non-complicated group, 60 hours (SD 38) in the perforated appendicitis group, and 176 hours (SD 107) in the intraabdominal abscess group (P = 4.6 x 10(-8)). No difference in intra-hospital delay was found. CONCLUSIONS: Children with complicated appendicitis are characterized by younger age, longer delay from symptom onset to correct diagnosis, and typical laboratory findings. Delays in diagnosis can be avoided by first considering the diagnosis of acute appendicitis in the differential diagnosis when examining any child with abdominal pain.