The dynamics in schoolchildren's health status varies in time, as the incidence of some chronic diseases decreases and that of the others increases. As a result, the health status of schoolchildren has deteriorated due to a reduction in the number of healthy children and an increase in the number of chronic patients in the past 30 years. Nervous, immune (allergic), and blood diseases have become frequent. Chronic ENT diseases have been encountered more infrequently due to the noticeable reduction in the incidence rates of tonsillitis and otitis. The positive trend is that schoolchildren have no rheumatism, infective allergic myocarditis, chronic pneumonia and diffuse glomerulonephritis and that the prevalence of locomotor disorders, renal and metabolic (obesity) diseases is low. During school time, the health status of children slightly improves due to the lower incidence rates of chronic diseases and the higher proportion of healthy children, but it has remained still worse than it was 30 years ago. The formation of chronic diseases has been found to occur in health group II children, who have morphological and functional changes, in 50% of cases from the first to the sixth forms and in 20% of cases from the seventh to the tenth forms. The present-day children are characterized by a combination of abnormalities and functional disturbances. This all require active and timely prophylactic measures to block chronization at the premorbid stage. Lifestyle (to keep the hygienic school regime, to do exercises and to go in for sports), sociohygienic conditions and genetic factors have profound effects on the health status.