Asthma severity and level of asthma control are two related, but conceptually distinct, concepts that are often confused in the literature. We report on an index of asthma control developed for use in population-based disease management. This index was measured on 5,181 adult members of a large health maintenance organization (HMO), as were various self-reported measures of health care utilization (HCU) and quality of life (QOL). A simple index of number of control problems, ranging from none through four, exhibited marked and highly significant cross-sectional associations with self- reported HCU and with both generic and disease-specific QOL instruments, suggesting that each of the four dimensions of asthma control represented by these problems correlates with clinically significant impairment. Qualitatively similar results were found for control problems assessed relative to the past month and relative to the past year. Asthma control is an important "vital sign" that may be useful both for population-based disease management as well as for the management of individual patients.