Using data from the Early Head Start Research and Evaluation Study, this study analysed the stability of child aggressive behaviour beginning in infancy and tested whether spanking when the child was 36 months was associated with aggressive child behaviour among three ethnic groups and whether maternal warmth moderated the effect of spanking on aggressive behaviour in each ethnic group at 36 months, after controlling for earlier aggressive behaviour. Participants included 693 Hispanic parent–child dyads, 1013 African-American dyads and 1086 Caucasian dyads who met qualifications for participation in the Early Head Start programme. Findings suggest that infant temperament was associated with aggressive behaviour at 24 and 36 months and that child aggression remained stable. Among the three ethnic groups, spanking was only associated with aggressive behaviour for children who had Caucasian mothers and maternal warmth did not moderate the effect of spanking on aggressive behaviour. Copyright © 2009 John Wiley & Sons, Ltd.