This study analyses the impact of improved water supply and sanitation on the level and rate of decline of child diarrhoea mortality in Stockholm 1878—1925. Previous studies have failed to demonstrate an effect of improved water supply on the risk of diarrhoea mortality at the individual level. Using data on access to water and sanitation from a household survey in 1895 and mortality rates and sociodemographic information from individual data 1878—1925 to analyse area differentials in diarrhoea mortality, it was found that the proportion having their own latrine in the household was associated with lower mortality risk in 1895—1900, while the proportion having water in the household was associated with lower diarrhoea mortality risk during the mortality decline until 1925. Population effects of improved water and sanitation on diarrhoea mortality may be better measured at area level than at individual level.