This study examines children's conceptual understanding and factual knowledge of the causes of cancer. Using a standardized, developmentally based, semistructured interview (ASK [AIDS (acquired immunodeficiency syndrome) Survey for Kids]), 784 children (43% black, 38% white, and 18% Hispanic; 48% female) in kindergarten through sixth grade attending six public elementary/middle schools in New Haven, Connecticut, were asked open-ended questions about the causes of cancer and, for comparison, the causes of colds and AIDS. Responses were scored for level of conceptual understanding and coded for factual content and factual accuracy. The level of conceptual understanding for causality of cancer increased consistently as grade level increased. When comparisons were made among the illnesses, children's level of conceptual understanding was significantly lower for the causes of cancer than for the causes of colds (p < .0001), but not significantly different from that of AIDS. Although the single most frequent cause of cancer mentioned was cigarettes/smoking (24%), more than one in five students stated that casual contact or contagion was a cause of cancer. More children cited causal contact/contagion than cited the following factually accurate or logically contributory causes combined: poor diet, air/water pollution or overexposure to sun, alcohol, and old age. Slightly more than one half of students in kindergarten through sixth grade worried about getting cancer, and the vast majority (80%) knew that cancer could be fatal. Children have a less sophisticated conceptual understanding of cancer than of colds and a very limited factual knowledge base for cancer, and thus they have the capacity to increase both their understanding and knowledge. These results have implications for the creation of developmentally appropriate cancer prevention curricula for elementary school-age children.