Optimal communication between providers of child health care and their patients depends on appropriate expectations of what the children can understand. Recently the developmental stages in children's understanding of the mechanisms of cause, prevention, and treatment of illness have been delineated. We studied the accuracy of clinicians' knowledge of these developmental stages. Pediatricians, nurses, and child development students were asked to estimate the age at which children made typical responses to five questions regarding illness mechanisms. Clinicians usually overestimated the age of younger children and underestimated the age of older children; they correctly estimated children's ages less than 40% of the time. Child health care providers might communicate more effectively with their patients if they became more familiar with typical stages in children's understanding of illness concepts.