Data from the National Health Interview Survey indicate that the prevalence of activity-limiting chronic conditions among children under age 17 years doubled between 1960 and 1981, from 1.8 to 3.8 per cent. Approximately 40 per cent of the overall rise in prevalence occurred before 1970. Most of the increase in prevalence during this early period can be attributed to changes in questionnaire design and aging of the child population following the "baby boom" years. The factors responsible for increases in reported cases of activity limitation following 1970 are more difficult to specify and evaluate. During this later period, the increase in prevalence was restricted to less severe levels of limitations. While prevalence levels rose for a variety of conditions during this period, respiratory conditions and mental and nervous system disorders demonstrated the largest changes. It appears that much of the increase in reported cases of activity limitations during the 1970s can be attributed to shifting perceptions on the part of parents, educators, and physicians.