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 < 0.05). The crude rate for stillbirths was highest in NL. The women in the northernmost area were socially less privileged and the localities less prosperous compared with those in the southernmost area. Significant spatial clustering of mean birthweights was found (P = 0.0016), with highest birthweight in the south-western part of the study area. A variable expressing the wealth of each locality, the financial capacity category (FCC), had its lowest mean value in NL, with a range of one to six for the localities studied here. A multilevel multiple regression model showed that, after allowing for sex, gestational age, mother's age, height and hypertensive disorders, parity, body mass index, previous low birthweight child and smoking as individual determinants of birthweight, part of the residual variation could be explained by the locality wealth parameter. Using the multilevel model, the differences in mean birthweight across the three latitude areas persisted but were reduced (difference OP vs. NL reduced from 105 g to 86.5 g). The relationship between birthweight and FCC was inverse U-shaped with the highest mean birthweight estimated for localities occurring in the middle of the range (FCC = 3). The wealthiest urban localities (FCC = 6) and the most deprived localities (FCC = 1) both had a predicted birthweight about 60 g below the maximum at FCC = 3, if all other factors were held constant. This result, taken together with the spatial clustering of birthweights, suggests that there may be important social and environmental determinants of birthweight that have yet to be identified.