Histopathological examination of tumours is of vital importance in clinical oncology as well as for epidemiological registration of cancer. Evaluation of malignancy is basic, but the reporting of tumour type is also necessary for studies of etiology. In addition to adequate epidemiological and statistical methods, quality assurance of histological diagnoses is of great value in such work. This paper contains brief comments on histological examination, epidemiological registration and epidemiological research, with special reference to cancer etiology, and giving some examples.