Of 6,099 children treated for malignancy, 16 (ages 3.5 to 18 years) developed acute appendicitis between 1962 and 1989. Fourteen had leukemia (ALL 10, AML 4). One each had rhabdomyosarcoma and Ewing's sarcoma. Active malignancy at diagnosis was noted in 10, 4 of whom had severe neutropenia (absolute neutrophil count less than 500/mm3). Of all the leukemics (2,794/6,099), abdominal pain during induction was a frequent complaint. The incidence of appendicitis, however, was low (0.5%). Nine of the 16 patients presented classically, facilitating prompt diagnosis and treatment. Six diagnoses were delayed. Three of these patients presented atypically with vague, nonlocalized pain, abdominal distention, lack of abdominal guarding, fever, dehydration, diarrhea, and unusual symptoms such as upper gastrointestinal bleeding. In each of these 6 patients the appendix was ruptured. Delays led to complications and deaths. Three patients required perioperative transfusions to treat excessive bleeding and two patients with ruptured appendicitis developed wound abscesses. Two patients died; in one, ruptured appendix was diagnosed only at autopsy. The other patient died of uncontrolled sepsis. Typhlitis occurring during induction chemotherapy may present similarly and is the main differential diagnosis. Typhlitis will usually improve with medical treatment alone. Nausea and vomiting (13/16), right lower quadrant pain (13/16), guarding (14/16), tachycardia (12/16), fever (10/16), and rebound tenderness (10/16) were the most frequent signs and symptoms of appendicitis. Persistent localized abdominal pain and guarding, lack of improvement with medical treatment, clinical deterioration, and the development of a mass were our indications for laparotomy. Despite major improvements in therapy, there is still a 37.5% error rate in our ability to accurately diagnose appendicitis in pediatric cancer patients.