BACKGROUND: Disability is prevalent among patients treated in Internal Medicine (IM), but its impact on length of inpatient stay (LOS) is unknown. Current systems of patient management and resource allocation are disease-focused with scant attention paid to functional impairment. Earlier studies in selected cohorts suggest that disability prolongs LOS. OBJECTIVE: To investigate the relationship of disability with LOS in IM, controlling for comorbidity. DESIGN: Prospective cohort study. PATIENTS: We charted 448 patients from an IM team admitted between 2008 and 2012 for sociodemographic, disease, biochemical and functional characteristics. Each IM team is on duty for one month annually, and patients were hence recruited for one month each year. MAIN MEASURES: Disability was measured using the Functional Independence Measure (FIM) recorded at discharge. Comorbidity was measured using the Charlson Comorbidity Index (CCI). KEY RESULTS: Of the 448 patients, 57.4 % were male with mean age 68.6 years. The mean LOS was 9.58 days. The mean motor and cognitive FIM scores were 57.1 and 25.7, respectively. The mean CCI score was 2.69. Thirty-four percent had major social issues impacting discharge plans. The five most common diagnoses for admission were pneumonia (8.9 %), urinary tract infection (7.8 %), cellulitis (7.6 %), heart failure (7.1 %) and falls (6.0 %). Both cognitive and motor FIM scores were negatively correlated with longer LOS (P < 0.001). On multivariate analysis, variables independently associated with longer LOS included the motor FIM score (P < 0.001), presence of social issues such as caregiver unavailability (P < 0.001), non-realistic patient expectations (P = 0.001) and administrative issues impeding discharge (P = 0.016). CONCLUSION: Disability predicts LOS in IM patients, and thus their comprehensive care should involve functional assessment. As social and administrative factors were also independently associated with LOS, there is a need to involve social workers and administrators in a multidisciplinary approach towards optimizing LOS.