Background: Critical care services underpin the delivery of many types of secondary care, and there is increasing focus on how to best deliver such services. The aim of this study was to investigate the impact of establishing a medical high dependency unit, in a tertiary referral center, on the workload, case mix, and mortality of the intensive care unit. Methods: Single-center, 11-year retrospective study of patients admitted to the general intensive care unit, before and after the opening of the medical high dependency unit, using interrupted time series methodology. Results: Over the duration of the study period, 3209 medical patients were admitted to the intensive care unit. There was a constant rate of medical admissions to the intensive care unit until the opening of the medical high dependency unit, followed by a statistically significant decline thereafter. There was a statistically significant decrease in the average severity of illness of medical patients prior to the opening of the medical high dependency unit, but there was no evidence of a change following the opening of the unit. There was no evidence of a statistically significant change in the estimated mean standardized mortality ratio for either medical or surgical admissions after the intervention. Conclusions: The opening of a medical high dependency unit had a minimal impact on the intensive care unit. There was, in all likelihood, an unmet need—of less seriously ill patients, who were previously looked after on a normal ward, but did not require intensive care unit admission—who are now cared for in the new medical high dependency unit. Interrupted time series analysis, although not without limitations, is a useful mean of evaluating changes in service delivery.