Background: Inpatient general medical units often look after older patients who have more complex co-morbidity including renal insufficiency. The consequences of renal insufficiency with respect to length of hospital stay (LOS) and mortality have not been well described in hospitalized general medical patients. Aim: To use a general medical inpatient population to evaluate the impact of reduced kidney function. Design: Retrospective cross-sectional study. Methods: We studied 504acute medical admissions through an Acute Assessment Unit between February and November 2007. Patients were classified as having chronic kidney disease (CKD), acute kidney injury (AKI), neither (control) or both. LOS, in-hospital mortality and post-discharge survival were evaluated. Results: Renal impairment was present in 151 patients. Ninety patients had CKD only and 61 had AKI with or without CKD. In-hospital mortality was increased in those with renal impairment compared with controls (9.3 vs. 3.4%; P = 0.006). Within 4 years of admission, 187 (39%) patients had died. Post-discharge mortality was significantly higher in all renal failure populations (hazard ratio: 2.57-4.38; P < 0.01). Adjustment for patient age, gender and Charlson index explained the increased mortality during and after hospital admission but did not explain increased LOS. Only a small proportion (13%) of admitted patients with renal insufficiency had renal disease documented in their discharge summaries. Conclusions: Many general medical inpatients (30%) have reduced kidney function at the time of admission. This study provides validation of the Modification of Diet in Renal Disease equation as a predictor of poor outcomes. Reduced renal function was associated with increased hospital LOS and mortality. Mortality rose with AKI and was explicable on the basis of the patients' age and co-morbidities. Renal insufficiency is documented infrequently in discharge summaries.