A family cancer database was constructed from the nationwide Swedish registries and includes approximately 6 million persons and >30,000 cancers in offspring diagnosed at ages 15-51 years and their parents. A particular advantage of the database is that the contribution of both parental lineages on cancer risk can be examined. Cancer risk in the offspring was increased approximately 1.1 times when the father had cancer, and no increase was noted when the mother had cancer. If both parents had cancer, the risk for sons was 1.4 and for daughters 1.3. The sites of increased cancer risk in the offspring were colorectum, breast, cervix, corpus uteri, ovary, testis, melanoma, eye, other endocrine glands, and multiple myeloma. The results among young and middle-age adults suggest that cancer in both parents increases the cancer risk in the offspring at many sites. The molecular genetic explanation may be that rare dominant single genes increase susceptibility at many sites, or that overlapping sets of genes control susceptibility at multiple sites.