Investigated the conceptions of illness and accuracy of understanding about their disease for children with juvenile rheumatoid arthritis (JRA). 54 children between the ages of 6 and 17 were interviewed individually about various aspects of JRA, with results suggesting that accuracy and illness conceptions could be reliably measured. As predicted, children's understanding about their disease followed a developmental progression, with older children demonstrating a more sophisticated understanding of JRA than younger children (significant differences between age groups on 3 of the 5 questions). Multiple regression analysis indicated that conceptual level (p < .001) was a better predictor of the child's accuracy of knowledge than was age (ns). Despite the developmental progression, there were a significant number of children functioning below the level expected for their age. In fact, the majority (75%) of children exhibited an understanding of JRA at the concrete operational level of cognitive development. The within-subject variability and striking misconceptions argue for ongoing evaluation of each child's understanding as a way to improve educational efforts.