Background: Population based data on risk of cancer in relatives of childhood cancer patients are sparse. Using linked population-based registries, we set out to evaluate risk of early onset cancer in first-degree relatives of childhood cancer patients. Methods: We queried the Finnish Cancer Registry and ascertained a cohort of 9135 individuals diagnosed with at least one cancer under the age of 21 years between 1970 and 2012. We then went on to identify a total of 58,211 unique first- and second-degree relatives by linking to the Central Population Registry. Relatives were then linked back to the annually updated Finnish Cancer Registry to identify cancer diagnoses in siblings, offspring and parents of childhood cancer patients, restricting to cancers occurring under the age of 40. Risk of cancer in relatives of the index case was estimated using standardized incidence ratios (SIRs) comparing cancer age and period specific incidence in relatives to that of the general population. Results: A total of 288 cancers were diagnosed in relatives during the 900,907 years of follow-up, while 266 cancers were expected. The overall risk of cancer in siblings of childhood cancer patients was elevated (SIR 1.18 95% CI 1.00-1.39). 144 of the childhood cancer patients were identified as having a sibling additional to index case with a diagnosis of cancer at age < 40; 44 of these 144 also had a parent with early onset cancer. The risk of early onset cancer was elevated in offspring overall (SIR 1.79 95%CI 1.05-2.81) and in offspring of retinoblastoma, malignant bone tumor and neuroblastoma patients. Siblings of lymphoma patients were at elevated risk of early cancer, and the mothers of 11 of 27 sibling pairs (lymphoma + cancer < 40 yo) also had cancer at age < 40. Conclusions: Linked registries allow family history of cancer to be evaluated across multiple relatives and to be longitudinally updated. Results are generally reassuring with regard to risk of cancer in relatives of childhood cancer patients. Elevated risk in relatives of retinoblastoma and malignant bone tumor patients are in line with the known cancer syndromes associated with these tumor types, and lymphoma and neuroblastoma families need further analysis.