This multilevel study of spatial variability in, and determinants of, birthweight was conducted using individual and ecological data in a geographically defined prospective birth cohort for 1986 in northern Finland. The study area comprises three large areas defined by latitude: Northern Lapland (NL), Southern Lapland (SL) and Oulu province (OP), comprising 74 localities with a total study population of 9216 singleton births. The mean birthweight was 3482 g for NL, 3537 g for SL and 3587 g for OP (NL vs. OP and SL vs. OP: P