The authors have investigated the morphological and morphometrical signs of thymus glands in prematurely born children died of different causes and worked out the algorithm of morphological signs coming along with this or that pathology. It was shown, that weight and weighting coefficient of the thymus gland correlate with degree of child prematurity and different causes of death. Drawing up the algorithm of the morphological changes to determine pathology of a thymus gland gave us an opportunity to establish criteria of normal development of a thymus gland and its developmental lagging, hypoplasia, dysplasia, premature atrophy against a background of incomplete pregnancy and different diseases. The data can be used as diagnostic criteria for evaluating morphological structure of the thymus gland of a newborn infant.