The authors compared the frequency of pregnancy and birth complications (PBC) from hospital records in antisocial adolescents and their same-sexed siblings who were relatively clear of antisocial behaviour. Probands, as a group, were not more likely to have had more PBC than their siblings, however, if there was a difference in complication frequency, it was usually the proband who was affected. The only effect of PBC within a pair was on the school performance and reading achievement of the proband. These results suggests that PBC may provide a marker for a subgroup of antisocial youth with a different etiology than similarly disturbed adolescents without PBC.