This study investigated the oral problems occurring in children receiving treatment for malignant disease at a regional oncology center. Forty-three children 2 to 14 years old were followed longitudinally from initial diagnosis for periods ranging from eight to 30 weeks for the development of oral and dental problems. Fifteen children had untreated decay; two required the removal of primary teeth before they began chemotherapy. Three children developed acute dental infections during treatment. Forty (93%) developed oral problems associated with their disease or treatment during the study period. Oral mucosal ulceration was the most frequently encountered problem; it was observed in 28 (65%) patients. In light of the high prevalence of dental and oral problems in these patients, this study emphasizes the need for positive dental involvement, both in pretreatment assessment and in the ongoing care of the pediatric oncology patient.