The reliability of criteria for diagnosis and differential diagnosis of Marek's disease and lymphoid leucosis of fowl, as part of herd diagnosis and on the basis of pathologico-anatomic findings as well as of pathomorphological and statistical tests has been checked for treatment in this paper. Tumorous lesions were macroscopically and histologically recorded from 1,261 hens of various industrialised poultry units. Marek's disease was established in 316 cases, lymphoid leucosis in 828 cases, mixed manifestations in 33 cases, and other neoplasias in 84 cases. These animals, together with 591 hens in which Marek's disease had been experimentally induced, were evaluated by pathologico-anatomic criteria of differential diagnosis. The conclusion was drawn that differential diagnosis of Marek's disease and lymphoid leucosis on the basis of pathologico-anatomic findings, without histological testing, was sufficiently reliable, when undertaken as part of herd diagnosis monitoring. The expected small number of false diagnoses will be unimportant to herd diagnosis. Both diagnosis and differential diagnosis will be conducted by the following criteria: age of animal, alterations caused by Marek's disease to nerves, eyes, and skin, tumour growth in bursa fabricii caused by lymphoid leucosis, atrophy of bursa fabricii caused by Marek's disease, as well as differences between both diseases regarding tumour development in other organs, with due consideration to be given to the number of affected organs, as a whole, and to pathologico-anatomic organic changes caused by tumour growth.