The purpose of this survey was to investigate the small-area variations in low birthweight within social services districts in Copenhagen and the relation of such variations to the socioeconomic characteristics of the district. The study was based on register data and included all live-born single on births to women living in the city of Copenhagen from 1987-90. We found a statistically significant association between district and risk of newborns being small-for-gestational age (SGA). This association was independent of adjustment for maternal age and parity. Only part of the association was explained, though this was still significant, using an index summarizing the socioeconomic living conditions in the district. Splitting up the index into its four constituent components, however, showed that the association between SGA and district was primarily due to housing conditions in the districts. Mapping the districts demonstrated a clear tendency of clustering of the high-risk areas with poor socioeconomic conditions.