Severity and vulnerability as important constructs in the Health Belief Model were examined in school-age children. Cognitive development theory and sex-role socialization theory provided the bases for predicting age and gender differences. Nine hundred forty-six children in first, fourth, and sixth grade from seven schools in five states participated in the study. Severity and vulnerability in six categories of health problems were described. Analysis of variance revealed significant age and gender differences. Girls perceived that health problems were both more severe and that they were more vulnerable to them than boys. Age was significant on the vulnerability dimensions, although age patterns varied by health problems. Clinical and educational implications are discussed.