People living close to nuclear power stations should take reassurance from the study, experts said.
The researchers found that children, teenagers and young adults who live near the two sites are not at an increased risk compared to the general population.
Their study, published in the British Journal of Cancer, examined cancer rates between 1963 and 2006 among those who were under 25 and living near Sellafield or Dounreay when diagnosed.
Lead author Kathryn Bunch, from the University of Oxford, said: "For many years, there have been concerns over the potential raised cancer risk among people - particularly children - who live near nuclear installations." She said the study showed there was not an increased risk.