INTRODUCTION: Human immunodeficiency virus (HIV)-infected patients in the current treatment era can achieve normal life expectancies but experience a high degree of medical and psychiatric comorbidity. Impaired physical function and pain, often in the context of mood disorders and substance abuse, are common in HIV-infected patients. The objective of this study was to investigate the relationship of pain, a modifiable condition, to functional impairment in HIV-infected patients, independent of mood disorders and substance abuse. METHODS: Participants in a prospective cohort of HIV-infected patients at the University of Alabama at Birmingham were included. Patient-reported outcome measures were used to cross-sectionally assess pain and physical function (EuroQOL), mood disorders (PHQ), and substance abuse (ASSIST). Univariate and multivariable models were built with pain as the principal independent variable of interest and three domains of physical function (mobility, self-care, and usual activities) as outcomes. Covariates included mood, substance abuse, age, race, sex, insurance status, HIV transmission risk factor, and CD4+ T-cell count. RESULTS: Among 1,903 participants, 693 (37%) reported pain; 509 (27%) had a mood disorder; and 157 (8.4%) reported current substance abuse. In multivariable models, pain was independently associated with increased odds of impairment in all three domains of physical function investigated-mobility (aOR 10.5, 95% CI 7.6-14.6), self-care (aOR 4.1, 95% CI 2.2-7.4), and usual activities (aOR 5.4, 95% CI 4.0-7.4). DISCUSSION: Pain was associated with substantially increased odds of impairment in physical function. Pain should be an important consideration in HIV primary care. Interventions to address pain and impaired physical function should be investigated.