A home measurement monitoring system has been developed for assessing progress and planning changes in care for patients with cystic flbrosis. Daily diary recording of specified measurements, quantitative symptom data, and free text are to be used for early detection of deteriorating trends before serious complications develop. Daily measurements made at home are lung capacity, body weight, breathing rate, and pulse. The program has been in place for the past two years, and has maintained a 75-80% consistent diary response rate among the 111 patients initially committed to the program. Measurements are easy to perform, equipment design is simple and rugged, and data handling routines designed for the program using the INSIGHT clinical data base system perform satisfactorily. Checking for data entry errors and validity checks of home measurements are a regular part of the data handling activity. Patient acceptance and long-term compliance in this program agrees very favorably with reports of other diary programs in chronic disease. Diary compliance was significantly greater among younger patients and those who lived long distances from the hospital. This study has demonstrated that home monitoring is a feasible program for patients with cystic fibrosis. It presents the possibility of detecting adverse health trends earlier than is now practical, so that patients can be treated before serious complications develop, thereby preventing the large fluctuations in health status that often accompany CF.