Purpose: Advancing age is associated with increased risk for acute admissions and readmissions. The societal challenges of ageing populations have made the prevention of readmissions come into focus. Readmission may be perceived as the result of inadequate treatment during index admission but may also be caused by the onset of new disease following a generally impaired health of geriatric patients. We aimed at comparing the diagnoses at index and readmission to illuminate this issue. Methods: This is a descriptive, retrospective cohort study of patients acutely admitted and readmitted (within 30 days from discharge) to the same geriatric ward (November 1, 2017–April 30, 2018). Electronic medical records were scrutinised manually for discharge diagnoses and patient characteristics. Results: Readmission rate was 10.7% (98 of 918 unique admissions). Mean age was 85.6 (men 56%). About 75% were readmitted with a new acute disease unrelated to index admission, most commonly pneumonia (27%), other infections (22%), and dehydration (14%). The health characteristics were long index length-of-stay (median 7; IQR 5–11), high Charlson Comorbidity Index (CCI ≥ 3, n = 49 (50%), polypharmacy (≥ 5 prescriptions) (94%), and hospitalisations 12 months prior to index admission (57%). Key conclusions: The majority of readmitted geriatric patients have contracted a new acute condition. Although being characterised by several adverse health characteristics, prospective studies comparing readmitted and non-readmitted geriatric patients are needed. Still, increasing the awareness of early recognition of acute disease onset in geriatric patients is warranted.