Fetal growth is an indicator of social inequalities in health that may have a long-term impact persisting into later life. Little is known about the social patterns of birth weight in Hong Kong. This is a study of live-born singletons from 1984 to 1997 in a Hong Kong birth registry. Ordinary least-squares regression and logistic regression are used to analyse birth weight and low birth weight (< 2500 g), respectively. A gradient of birth weight and prevalence of low birth weight is demonstrated according to mothers' educational attainment. In relation to babies of the most educated mothers, babies of the least educated mothers had a mean deficit of 46g in birth weight and an odds ratio of 1.56 of low birth weight (each P<0.05). This social gradient was hidden unless parity was adjusted for. Unexpectedly, migrants from mainland China delivered heavier rather than lighter babies (each P<0.05). Type of living quarters and parental relation were also related to birth weight and low birth weight (each P<0.05). Continuous monitoring of the social patterns of birth weight is recommended.