e18751 Background: In the last years, Immunotherapy (IT) has emerged as a standard treatment in an increasing number of tumors. This type of treatment has a specific toxicity profile which is clearly different from chemotherapy, known as an Immuno-related Adverse Event (AEir). We know the data from clinical trials, but little about the incidence and impact of this EAir in our clinical practice. Methods: A retrospective observational study was carried out including all patients from our institution (HUPHM in Madrid) who had received IT, either in monotherapy or in combination between January 2014 and December 2019. A total of 279 patients were included and data were collected between January and July 2020, guaranteeing a minimum 6-month follow-up after receiving the first dose of immunotherapy. The toxicities found were classified into four categories: pulmonary, digestive, endocrine and others, and have been graded according to CTCEA v.5 (Common Terminology Criteria for Adverse Event) published in November 2017 and analyzed according to drug and tumor. Results: The most frequent diagnoses in our patients were: 60% lung carcinoma, 15% melanoma, 8% kidney carcinoma, and 6% bladder carcinoma. 76% of the patients received IT as first or second line in a metastatic context, 6% in the initial stage (clinical trials) and the rest in more advanced lines of treatment (3 or more). 67% received anti-PD1 drug, 6% anti-PDL1, 4% anti-CTL4 monotherapy, 10% a combination of several IT drugs, and 14% an IT combination and chemotherapy. 45% of the total presented EAir (16% grade I, 14% grade II, 11% grade III and 4% grade IV). 1/5 of the patients had manifestations in more than one organ. The incidence of the different toxicities in our population was listed in the table below. These patients reported 8% dermatological toxicities, 6% had renal toxicity (most of them grade III or IV), only 2% had arthralgia or myalgia, and 3% asthenia. Combined IT treatment had significantly higher rates of pneumonitis, colitis, and endocrine toxicities. These differences were not observed between the monotherapy treatment and the combination of immunotherapy plus chemotherapy. Conclusions: Immunotherapy has represented an important advance in oncology, achieving long survivals in a growing group of tumors. Immunotherapy has a unique toxicity profile that is very different from chemotherapy and with which we must become familiar. Most of the adverse events are mild and if they are diagnosed early and with the appropriate treatment, maintenance of IT is possible. Severe toxicity (III-IV) means in most cases the suspension of treatment, compromising its efficacy. Therefore, we must learn to recognize these toxicities early and apply the recommended treatments as soon as possible.[Table: see text]