BACKGROUND: Mounting evidence suggests that loneliness increases the risk of poor health outcomes, including cardiovascular disease and premature mortality. Objective: This study examined the prevalence of loneliness in an urban, underserved family medicine residency clinic and the association of loneliness with health care utilization. METHODS: Adult patients (N = 330; M age = 42.1 years, SD = 14.9; 63% female; 58% African American) completed the 3-item UCLA Loneliness screener at their primary care visits between November 2018 and January 2019. A retrospective case-control study design was used to compare health care utilization [hospitalizations, emergency department (ED) visits, primary care visits, no-shows and referrals] in the prior 2 years between patients who identified as lonely versus those who did not. Covariates included demographics and clinical characteristics. RESULTS: Nearly half (44%) of patients exceeded the cut-off for loneliness. Patients who were lonely were more likely to identify as African American, have depression and have a substance use disorder. Patients in the lonely group had significantly longer hospital stays and more primary care visits, no-shows and referrals than patients in the non-lonely group; there were no differences in number of hospitalizations or ED visits. CONCLUSIONS: The prevalence of loneliness in an urban, underserved primary care clinic was much higher than prior prevalence estimates in primary care. Patients who are lonely may use more health care resources than patients who are not lonely. Primary care may be an ideal setting in which to identify patients who are lonely to further understand the impact of loneliness on health care outcomes.