Records of 1049 cases of Sudden Infant Death Syndrome (SIDS) (100%) in the Sydney metropolitan area (SMA) from 1980 to 1989 were analysed in relation to ambient temperature, geographical distribution and socio-economic scale. The SIDS rate varied between eastern and western Sydney and between statistical subdivisions; it peaked sharply in July, coinciding with the monthly mean minimum daily temperature (MMMDT; 6.5 and 5.4 degrees C, respectively). The inverse linear relationship between the SIDS rate and the MMMDT for the regions and the subdivisions was highly significant (P < 0.001). The SIDS rate showed a significant inverse linear relationship to two socio-economic scales (allotment and house value) for 82 of the 87 suburbs (94%) of the SMA. Temperature, indicative of cold weather, was the major factor determining SIDS rates, and this, together with socio-economic factors, largely explains the observed geographical distribution of SIDS rates in the SMA.