Misdiagnosis and under-detection of delirium may occur in many medical settings. This is important to address as delirium clearly increases risk of morbidity and mortality in such settings. This study assessed whether Veterans who screened positive on a delirium severity measure (Memorial Delirium Assessment Scale; MDAS) differed from those with and without corresponding medical documentation of delirium in terms of cognitive functioning, psychiatric/medical history, and medication use. A medical record review of 266 inpatients at a VA post-acute rehabilitation unit found that 10.9% were identified as delirious according to the MDAS and/or medical records. Of the Veterans who screened positive on the MDAS (N = 19), 68.4% went undetected by medical screening. Undetected cases had a higher number of comorbid medical conditions as measured by the Age-Adjusted Charlson Index (AACI) scores (median = 9, SD = 3.15; U = 5.5, p = .003) than medically documented cases. For Veterans with a score of 7 or greater on the AACI, the general relative risk for delirium was 4.46. Delirium is frequently under-detected in a post-acute rehabilitation unit, particularly for Veterans with high comorbid illness. The relative risk of delirium is up to 4.46 for those with high medical burden, suggesting the need for more comprehensive delirium screening in these patients.