Background: In 2015, Regional Hospital Silkeborg in Denmark introduced a 24-h telephone access hospital outpatient clinic (24-h access clinic). The 24-h access clinic allowed patients to call the hospital outpatient clinic in case of an exacerbation of symptoms. The aim of this study was to evaluate the effectiveness of a 24-h access clinic in terms of healthcare utilisation and mortality in patients with chronic conditions.Methods: This study was designed as a registry-based controlled cohort study. The 24-h access clinic was established at Silkeborg Regional Hospital in Central Denmark Region, and the five other regional hospitals served as comparison hospitals. We included patients in hospital outpatient follow-up with chronic obstructive pulmonary disease, congestive heart failure, atrial fibrillation/flutter, inflammatory bowel disease and chronic liver disease. Outcomes were hospital admissions, length of stay (LOS), intensive care unit assistance and outpatient visits, contacts to general practice, municipal home nursing and all-cause mortality. Follow-up was 18 months.Results: The study included 992 24-h access patients and 3,878 usual care patients. For the five conditions combined, the 24-h access patients had fewer all-cause admissions (incidence rate ratio (IRR) = .81, 95% confidence interval (Cl) = .71 to .92), general practice out-of-hours contacts (IRR = .81, CI = .71 to .92) and shorter LOS (IRR = .71, CI = .57 to .88), but the rate of all-cause outpatient visits was higher (IRR = 1.07, CI = .99 to 1.15). General practice daytime contacts were similar between the groups, and there was no significant difference in mortality.Conclusions: A 24-h access clinic reduced acute admissions and LOS compared to usual outpatient care. Planned outpatient visits increased and substituted unplanned acute care, thus enhancing integration of care by allowing for access to the correct level of care and ensuring continuity of care.