Acute interstitial nephritis (AIN) corresponds to a decline in kidney function due to an injury induced by drugs (in the majority of cases), infections, and autoimmune disorders. It is characterized by the presence of an interstitial inflammatory infiltrate in the kidney. Here, we describe a case of a man with a previous medical history relevant to ulcerative colitis (UC) who was admitted due to acute kidney injury (AKI) in the setting of AIN secondary to mesalazine.