Background. Certain types of cancer in children and young adults have
been linked with an increased risk of cancer in close relatives. Howev
er, the relation between childhood cancer and familial risk remains to
be fully assessed in population-based studies. Methods. We conducted
a nationwide study in Denmark of 11,380 parents of children with cance
r. The children were identified from records in the Danish Cancer Regi
stry; their parents were identified from population registers. The occ
urrence and rate of cancer in the parents were determined with use of
the Cancer Registry's files and compared with national incidence rates
for various categories of tumor. Results. Overall, 1445 cancers were
diagnosed in the parents, as compared with 1496 expected from national
incidence rates, to yield standardized incidence ratios of 1.0 (95 pe
rcent confidence interval, 0.9 to 1.0) for all parents, 1.0 for mother
s, and 0.9 for fathers. The lower rate of cancer among fathers reflect
ed their lower standardized incidence ratio for lung cancer (0.8; 95 p
ercent confidence interval, 0.6 to 0.9), as calculated from 114 observ
ations. Conclusions. Genetic determinants are important in several typ
es of childhood cancer, but the genetic susceptibility to tumors does
not generally extend to the parents of children with cancer, nor do th
e patterns of incidence point to the influence of shared environmental
factors. Thus, cancer in children should not be viewed as a general m
arker for an increased risk of cancer in the patients' parents.