Few studies reported in the literature have addressed the long-term trend of the use of medical care for people with intellectual disabilities (ID) in institutions. The subject cohort in this study was made of 168 individuals with ID in a public residential facility from 1999 to 2002 in Taipei, Taiwan. The average age of participants was 19.3 years, and their average stay in an institution was 6.6 years. The average annual outpatient visit of the study participants was 18.2 in the previous 4 years. It was found that they had more medical visits than the general population. Nearly 20.8–34.5% (average 29.0%) of the participants utilized more than 25 visits annually which was defined as high outpatient visit users. This group of high outpatient users consumed more than half of the total annual outpatient care visits in the past 4 years. In the full model of Generalized Estimating Equations to compare the high and non-high outpatient users, the factor of individuals with ID dwelling in the institution were more likely to be high outpatient care users than individuals who were only accepting institutional day care services (OR = 6.29, 95% CI = 1.35–29.30). The present study provides general information of high outpatient utilization and its determinants of people with ID and provides evidence for medical care decision makers dealing with policy development for people with ID care in institutions.