Whether cancer predisposing familial factors are associated with childhood tumors is unclear. The purpose was to study the incidence of childhood and adult tumors in extended families of children with cancer. Family history of cancer was obtained through questionnaires, and the Swedish Population-, and Cancer Registries for 194 childhood cancer patients aged ≤18 years, diagnosed 1972-2009. Standardized cancer incidence ratios (SIR), and 95% confidence intervals (CI) were estimated and compared with expected rates. Overall, 21 of the 194 patients had any relative with a childhood tumor. When restricted to first- to third degree relatives, increased incidences of childhood (SIR: 2.5; 95% CI: 1.3-4.4) and adult tumors (SIR: 1.5; 95% CI: 1.3-1.7), especially in the prostate (SIR: 2.7; 95% CI: 1.9-3.8) and breast (SIR: 1.7; 95% CI: 1.2-2.4) were observed. Prostate and breast cancers were observed at earlier than average ages. No TP53 mutations or known cancer predisposing syndromes were found in families with multiple childhood tumors. Familial factors may increase the risk for childhood cancer and modify the age of onset of common adult tumors. Studying extended families with multiple childhood tumors may be a valuable approach to understanding the etiology of childhood tumors.