Sudden unexpected death due to clinically undiagnosed neoplasia in infancy and childhood (SUDNIC) is a rare phenomenon, with only small numbers of cases reported in the literature. In the majority of instances, the tumors involve critical structures within the heart or central nervous system and include gliomas, medulloblastomas, rhabdomyomas, and neoplasms of stromal elements. A 20-year retrospective review of autopsy records from the Hospital for Sick Children, Toronto, was performed (1984-2003, n = 4926), and 7 cases of SUDNIC were identified (0.14%). In addition, 1 case was obtained from the files of the Children's and Women's Health Centre of British Columbia, Vancouver. Diagnoses included 2 cases of acute leukemia (1 myelogenous, 1 lymphoblastic), 2 cases of mediastinal lymphoblastic lymphoma (pre-T cell type), 1 papillary fibroelastoma of the mitral valve prolapsing into and totally occluding the left anterior descending coronary artery, 1 medulloblastoma, 1 Wilms tumor associated with fatal intraperitoneal hemorrhage, and 1 widely disseminated gastric carcinoma. These cases demonstrate that infants and children may have minimal or no symptoms in the presence of significant disease and highlight the need for a thorough autopsy examination in cases of sudden unexpected death in infancy and childhood.