The growing utilization of hospital care, especially of the services of Emergency Departments (ED), has been of great concern for many Western countries. The purpose of this study was to relate the amount of hospital care utilization to the frequency of ED visits. The study, based on a computerized medical information system, was carried out at Huddinge hospital which serves a suburban area of Stockholm, Sweden. ED visits were found to predict hospital care utilization in a 5-year follow-up of a 10% population sample. Persons who had made 2 or more ED visits during a period of 15 months before follow-up (less than 1/8 of the population sample) contributed 24% of all hospital outpatient visits, 29% of all hospital admissions and 31% of all hospital days during the 5-year follow-up period. The number of hospital outpatient visits per 100 personyears was more than 3 times higher among those who had 4 or more ED visits as compared to those who were non-visitors at the ED. Hospital admissions and days were 5 times higher. The utilization of medical specialties differed most in psychiatry, where those with 4 or more ED visits had nearly 17 times more admissions per 100 personyears than non-visitors. The difference increased gradually with increasing number of ED visits and was also evident in each age group. The mortality was also significantly increased for those with several ED visits. Higher migration in the latter group may besides indicate social instability.(ABSTRACT TRUNCATED AT 250 WORDS)