The aquaculture industry is growing and includes the farming and breeding of more than 580 aquatic species worldwide. The rainbow trout (Oncorhynchus mykiss, Walbaum 1792) is the most commonly bred trout species in Hungary. As broodfish form the basis of most fish farms, investigation into tumours occurring in trout, although under-researched, has proven to be a valuable and necessary field of study. During our investigation, we examined a broodstock of 3- to 6-year-old rainbow trouts (800) affected with idiopathic intestinal tumours (3%) which had consequentially led to ileus (40%). While performing necropsy, initial pathological observations showed intussusceptions. Tumours were discovered upon opening the body cavity, as well as metastasis forming in the livers and in the vessels of the gills. Histopathological and immunohistochemical tests allowed us to identify the neoplasms. The primary adenocarcinoma was found to have been developed within the intestines of the fish. The tumour tissue broke through the basal membrane and infiltrated the propria, protruding asymmetrically into the lumen of the mid-intestines, causing it to narrow significantly. This subsequently led to passage disorders, invagination of the intestinal segment and finally the emaciation of the fish. Histopathological and immunohistochemical inspection of the tumour cells displayed a high mitotic index, confirming malignancy.