This study used a detailed home observational system to analyse mother-child interaction during spontaneous joint play. Compared to matched controls, mothers of preschool children with behaviour problems initiated a smaller proportion of activities, made fewer contributions to keeping the activity going, and were less responsive to the child's contributions. In contrast to controls, problem children played a stronger role than their mothers in initiating and maintaining activities. Less positive affect was shown in the problem group, and play more frequently degenerated into conflict. The findings imply that low maternal involvement in play may be an important factor in the development of behaviour problems. Intervention studies are needed for further verification.