The purpose of this study was to show the differences in the mortality rates of children from Catholic and Lutheran families in 19th century Poznań, and to elucidate the causes of these differences. Data from Catholic and Lutheran parish death registers were used. The infant death rate (IDR), neonatal and postneonatal death rates and life table biometric functions were calculated and causes of deaths were characterized. The worst child mortality values (IDR=394.4; neonatal and postneonatal death rates, respectively, 117.1 and 277.4; e0=16.14 years; Crow's Index=2.47) were obtained for the poor Catholic Parish of St Margaret. The lowest infant and neonatal and postneonatal death rates were observed to have occurred in the Catholic Parish of St Maria Magdalena situated in the city's more affluent central area (mortality rates, respectively, 269.9, 93.1 and 176.9; e0=24.63 years; Crow's Index=0.96). The widest range of differences with regard to death rates was found for the Lutheran Parish of St Cross (the infant, neonatal and postneonatal death rates were, respectively, 293.1, 99.1 and 193.9; e0=28.03 years; Crow's Index=0.92). The St Cross Parish encompassed a fairly large area of the city characterized by varying ecological conditions. Among infants and young children from the three studied populations a high frequency of deaths due to infectious diseases, diarrhoeas, dysenteries and tuberculosis were observed. Differences in the mortality of children from Catholic and Lutheran families in 19th century Poznań resulted from ecological conditions, among which water played the most important role, rather than from religious differences.