Concern about the neurotoxicity of lead, particularly in infants and young children, has led to a revision of blood lead levels which are considered to involve an acceptable level of human exposure. Drinking water guidelines have also been reviewed in order to reduce this source of population exposure to lead. In the last 20 years, guidelines have been reduced from 100 to 50 to 10 microg/litre. Lead in tap water used to be a major public health problem in Glasgow because of the high prevalence of houses with lead service pipes, the low pH of the public water supply and the resulting high levels of lead in water used for public consumption. Following two separate programmes of water treatment, involving the addition of lime and, a decade later, lime supplemented with orthophosphate, it is considered that maximal measures have been taken to reduce lead exposure by chemical treatment of the water supply. Any residual problem of public exposure would require large scale replacement of lead service pipes. In anticipation of the more stringent limits for lead in drinking water, we set out to measure current lead exposure from tap water in the population of Glasgow served by the Loch Katrine water supply, to compare the current situation with 12 years previously and to assess the public health implications of different limits. The study was based on mothers of young children since maternal blood lead concentrations and the domestic water that mothers use to prepare bottle feeds are the principal sources of foetal and infant lead exposure. An estimated 17% of mothers lived in households with tap water lead concentrations of 10 microg/litre (the [WHO,] guideline) or above in 1993 compared with 49% in 1981. Mean maternal blood lead concentrations fell by 69% in 12 years. For a given water lead concentration, maternal blood lead concentrations were 67% lower. The mean maternal blood lead concentration was 3.7 microg/litre in the population at large, compared with 3.3 microg/litre in households with negligible or absent tap water lead. Nevertheless, between 63% and 76% of cases of mothers with blood lead concentrations of 10 microg/dl or above were attributable to tap water lead. The study found that maternal blood lead concentrations were well within limits currently considered safe for human health. About 15% of infants may be exposed via bottle feeds to tap water lead concentrations that exceed the WHO guideline of 10 microg/litre. In the context of the health and social problems which affect the well-being and development of infants and children in Glasgow, however, current levels of lead exposure are considered to present a relatively minor health problem.