In Italy, the COVID-19 epidemic curve started to flatten when the health system already exceeded its capacity, raising concerns that the lockdown was indeed delayed. Aim of this study was to evaluate the health effects of late implementation of the lockdown in Italy. Using national data on daily number of COVID-19 cases we first estimated the effect of the lockdown, employing an interrupted time series analysis. Second, we evaluated the effect of an early lockdown on the trend of new cases, creating a counterfactual scenario where the intervention was implemented one week in advance. We then predicted the corresponding number of intensive care unit (ICU) admissions, non-ICU admissions, and deaths. Finally, we compared results under the actual and counterfactual scenarios. An early implementation of the lockdown would have avoided about 26126,000 COVID-19 cases, 54,700 non-ICU admissions, 15,600 ICU admissions, and 12,800 deaths, corresponding to 60% (95%CI: 55% to 64%), 52% (95%CI: 46% to 57%), 48% (95%CI: 42% to 53%), and 44% (95%CI: 38% to 50%) reduction, respectively. We found that the late implementation of the lockdownin Italy was responsible for a substantial proportion of hospital admissions and deaths associated with the COVID-19 pandemic.