Medical Officer of Health reports for London boroughs, 1900-19, are analysed to determine the incidence of neonatal breast-feeding, duration of lactation, reasons for early supplementation and premature weaning, and their relationship with infant mortality. In a sample of 222,989 infants, breast-feeding rates were very high. Over 90% were breast-fed in the first month, almost 80% at 3 months, and over 70% at 6 months. The poorest boroughs had the highest rates of neonatal breast-feeding, but also a higher incidence of early supplementation and premature weaning. There was a significant positive correlation between breast-feeding and infant mortality due to poor maternal diet and health, poor nutritional quality of supplementary and substitute foods, and the use of an unhygienic feeding bottle. Energetic campaigns to counteract these problems probably contributed to the fall of infant mortality in this period.