This paper provides basic epidemiological data on smoking and drinking patterns before pregnancy amongst 1117 pregnant women attending a London antenatal clinic. Similar data are already available from North America and surveys have been done in Scotland but there is a dearth of information from England and Wales. In the early stages of pregnancy, prior to recognition, heavy alcohol consumption (in excess of 56 units of alcohol per week) is associated with the Foetal Alcohol Syndrome. More moderate levels of intake (as little as 10 units of alcohol per week) have been associated with Foetal Alcohol Effects such as growth retardation. In our sample 19% of the mothers were drinking more than 10 units of alcohol per week. Six per cent were consuming more than the 14 units of alcohol currently recommended by three of the medical Royal Colleges as 'safe' for non-pregnant women. Average weekly alcohol consumption was related to both the usual frequency and amount drunk, but also to binge drinking, beverage choice and combination, and reasons for drinking. In general, drinking was frequent and light. Those women who drank more per week tended to drink more frequently than those in the lower weekly consumption bands. Fourteen per cent of the sample admitted to binge drinking (more than 14 units of alcohol in a single sitting). Wine was the most popular beverage type. The range of beverages consumed was wider in the higher alcohol consumption bands. Those in the lower bands were more likely to drink only one beverage type. Social reasons for drinking were of far greater importance than reasons which emphasised the effects of drinking. However, heavy drinkers were unlikely to drink solely for social reasons. Seventy-seven per cent of the mothers were non-smokers, but the incidence of smoking increased in the higher alcohol consumption bands. The demographic characteristics of the smokers were in marked contrast to those of the heavier drinkers. Smokers tended to be younger, of lower social status and multiparous, whereas the heavier drinkers were more likely to be older, of higher social status and primiparous. This finding is of fundamental importance to those interested in planning screening or intervention programmes.