The objective of the study was to estimate and compare the correlation coefficients of head circumference and weight at birth among sibling pairs. Pairs of singleton siblings were ascertained among children born in Norway to the same mother between 1978 and 1997. Head circumference, birthweight and other perinatal factors were registered in the Medical Birth Registry of Norway. Head circumference measurements were obtained for first- and second-born in 287 448 sibling pairs. The correlation coefficient of head circumference among first- and second-born siblings was 0.343 (standard error 0.002) compared with 0.477 (standard error 0.002) for birthweight. These results were similar for later-born sibling pairs and only slightly influenced by other factors such as sex, fatherhood and time between pregnancies. Sensitivity analyses showed that substantial measurement error is required to explain the lower correlation for head circumference. In conclusion, the sibling correlation of head circumference was consistently weaker than that of birthweight.