Symptoms are the outward manifestations that allow children to identify and recognize illness; children's understanding of the symptoms of an illness may be directly related to their understanding of its cause or means of transmission. This study is the first empirical investigation of children's conceptual understanding and factual knowledge of the symptoms of AIDS. Children (N = 361; grades K to 6; 57% black, 24% Hispanic, 19% white; 52% female) attending four public schools in New Haven, Connecticut, were interviewed using a standardized semistructured interview (ASK, AIDS Survey for Kids) that included open-ended questions about the symptoms of AIDS and, for comparison, cancer and colds. Responses were scored for level of conceptual understanding and coded for factual content. For each illness, grade level was the variable most strongly correlated with symptomatology concept score (R = .42-.48, p < .0001) and contributed significantly (p < .0001) to the variance observed in concept score even after controlling for race, gender, verbal fluency, and socioeconomic status. The mean concept score was lower (p < .01) for symptomatology of AIDS (2.8 of possible 5) than for cancer (3.1) or colds (3.9). In addition, far more symptoms were named for colds than for either cancer or AIDS. Children who believed that HIV is spread via each of five potential means of transmission by casual contact were more likely (p < 01) to cite cold symptoms as symptoms of AIDS. We conclude that there exists a developmental progression in children's understanding of the symptomatology of AIDS. Children have a less sophisticated conceptual understanding and narrower factual knowledge base for AIDS than for colds and therefore have the capability to increase their understanding and knowledge about AIDS. Furthermore, improving children's understanding of the symptoms of AIDS may diminish misconceptions about transmission of HIV via casual contact.