A scheme of pathogenesis of thymomegaly based on the laboratory findings of 575 children with thymus enlargement, was drawn up to prove that thymus enlargement was not constitutional abnormality in children but thymus pathology. Routine clinical methods, investigation of the body proportions, indices of immunity and adrenocortical and thyroid function were used. Disorder of thymus function and associated changes in the state of some endocrine glands were proved. The clinical picture of thymus enlargement in children was like this: lymphoproliferative and endocrine-metabolic symptoms, compression of the vital organs by the thymus, immunity disorders.