Heikkinen SMM, et al. - Among relatives of a population-based series of early-onset (≤ 40 years) cancer patients in Finland, researchers assessed familial aggregation of cancer in this registry-linkage study. Overall 376,762 relatives and 40,538 families were included. Familial aggregation was assessed for early-onset breast, colorectal, brain and other central nervous system (CNS) cancer and melanoma. In early-onset colorectal and CNS cancers, high familial aggregation was revealed. In multiplex families with CNS cancers, neurofibromatosis was primarily responsible for familial aggregation, and familial adenomatous polyposis- and hereditary nonpolyposis colorectal cancer-syndromes mainly accounted for familial aggregation of colorectal cancer. Very early exposure to environmental factors and/or rare genetic factors could be supported by the observed pattern of familial aggregation of early-onset breast cancer.