The article considers child mortality of the Hetmanschyna population of the ХVІІІ-th century. The primary source of research is the Church metrical book of the Nativity of Christ found at the small town Yaresky of Myrhorodskyi regiment. With the application of historical and demographic methods and approaches, the basic mortality rate and its features are determined. It was revealed that in the Hetmanschyna only the postnatal infant’s mortality rate was recorded, but neonatal mortality was almost not taken into account. Even considering such a misreporting, children between the ages of 0-14 constituted a part of more than half of all the dead ones recorded in metric books. This fact corresponded to the «paradox of infant mortality», according to which, the mortality rate declined sharply with each passing year until reaching the age of 5 years. Actually totally 460 children out of the thousand baptized infant survived and reached 5 years of age. The highest mortality rate was in the first years of life, but the exact life duration determination is excluded and become impossible by the fact that when recording the age of all children at the age of several days, weeks or months were recorded as those who were one year old. Thus, under this marker, two one-year age groups are accrued. This makes impossible to calculate accurately the infant mortality rate. The obtained demographic indicators evidenced that infant mortality had a strongly pronounced seasonal prevalence. April was the most dangerous month for children, which was explained by communicable diseases burst. In addition, in April, the challenges with food commenced, and malnutrition weakened the body’s resistance. In the first instance, it concerned children who had not been breastfed anymore. On the other hand, famine weakened the mothers who were giving birth at that time. During heavy yield summer months, the infant mortality raised again from June to August. This month is the second highest by mortality rate in Yaresky. The warm year season traditionally contributed to diseases of the digestive tract, food poisoning, intestinal infections. The August’s peak of mortality can be explained by the deterioration of drinking water quality, as well as the engagement in the fieldwork, which affected and worsened childcare. Autumn’s mortality outbreak was caused by infectious diseases, which rapidly spread due to the microclimate of the living space. In general, such mortality was inherent in an agrarian, traditional society that was not introduced to professional medicine and health care. У статті вивчається дитяча смертність в населенні Гетьманщини ХVІІІ ст. Основним джерелом дослідження є частина про «померлих» метричних книг церкви Різдва Христового містечка Яреськи Миргородського полку. За допомогою історико-демографічних методик встановлено основні коефіцієнти смертності, її характеристики. На основі даних коефіцієнтів висловлено припущення про основні причини смертності, чинники, що впливали на її коливання. Встановлено, що дитяча смертність мала яскраво виражену сезонність, що визначалася особливостями клімату, температурними коливаннями, забезпеченістю їжею та питною водою, спалахами інфекційних хвороб та мікрокліматом житлового простору. Загалом така смертність була притаманною аграрному традиційному суспільству, що не знало професійної медицини та медико-санітарного догляду.