Research on children's knowledge of illnesses has largely concentrated on studying how children reason about common innocuous diseases. It is also important to uncover how children reason about more severe diseases, such as cancer, to be able to treat and communicate with children diagnosed with this disease. Several aspects of prevalent childhood cancers may challenge the intuitive theories that children hold about illness and can make cancer a difficult illness for children to understand. In the present study we assess knowledge of six dimensions (prognosis, internal, course, contamination, contagion, cause) of cancer and colds as a comparison illness. Healthy 5-, 7- and 10-year-olds, and adults were administered a yes/no and forced-choice questionnaire created to tap into six dimensions of two illnesses. Results indicate that 5-year-olds reason about cancer and colds in similar ways, but 7- and 10-year-olds begin to make a distinction between cancer and colds on some of the illness dimensions. Children in the youngest two age groups were found to think that cancer is just as contagious as colds but by age 10 children begin to think of cancer as a less contagious illness. Adults clearly differentiate between the two illnesses on almost all the dimensions. (Contains 2 tables.)