The paper wants to present the data of infection of the Health Care Workers of a research and teaching hospital in Milan, Italy . The majority (2554 , 55.9 %) of 4572 HCWs were tested for SARS-CoV-2 and 8.8% were found positive . Most of the tested workers were women, but we found higher relative frequency of positivity for men, even after adjustment for age, working area, and occupation . The higher frequency of positive tests in the medicine area is probably explained by the higher concentration in that area of COVID-19 patients . Conversely, the low frequency of positive HCWs in intensive care units is probably explained by the diffuse and continuous use of PPD . Our results show that HCWs in a research and teaching hospital in the most hit Region in Italy had a similar pattern of infection as all other HCWs all over the world . The problem of SARS-CoV-2 infections among the hospital personnel HCWs should remind us the concerns about hospital acquired infections both for patients and HCWs.