The authors describe the results of observation and clinico-laboratory examination of patients during seasonal rises of the rotavirus infection incidence in 5 districts of the Moscow region in 1984-1987. The diseases ran their courses in the form of gastroenteritis and enteritis and were marked by a great number of the grave patterns (42-46%), mainly in children of the first three years of life, by a high percentage (77.5%) of the aggravated premorbid status of patients with the grave patterns. The rotavirus nature of diarrheas was supported by the data of direct electron microscopy, ELISA, and solid-phase coagglutination test. The symptom-complex of the clinical manifestations of rotavirus infection was characterized in detail. The histologic and morphometric data pertaining to two lethal outcomes were discussed. It has been demonstrated that respiratory viral infections, primarily influenza, produced an adverse effect, promoting the formation of the severe patterns and the onset of unfavourable outcomes of rotavirus infection in children.