Behavioural problems, such as defiance and aggression, are the most common problems presented by children in mental health services. Intervention in these problems, however, failed to develop clinically significant changes in almost one third of the children referred to in the literature. Study 1 was designed to evaluate the effect of Child Parent Relationship Therapy (CPRT) and Level-4 Positive Parenting Programme (Standard Triple P) on children (aged 4-6) with behavioural problems in four families, using a single-subject design. Results indicated that CPRT promoted high levels of parent engagement and increased positive parenting skills, and the therapy reduced child behavioural problems in two families. In the light of the findings, it was decided to investigate CPRT further with a particular examination of whether the gains observed early in the therapy meant that a shorter duration programme may be as effective. The original 10-session CPRT was revised and a manual for a 5-session adapted CPRT was developed by incorporating three key components identified as effective in the intervention. These were (a) deconstruction-and-reconstruction of parents’ negative attribution of child behaviour problems; (b) the basic child-centred play strategies; (c) two home visits, one for a live demonstration of child-centred play sessions and another for the supervision and coaching of parent-child play sessions. Study 2 was designed to evaluate the effects of the adapted CPRT on eight parent-child dyads using multiple-baseline single-subject designs and to explore its effects within each family. Observation data revealed a clear relationship between training in intervention strategies and changes in parent behaviour, which resulted in beneficial changes in child behaviour. Visual analysis indicated improvements in targeted child positive and negative behaviours across all children. The pre- and post-intervention analyses suggested that adapted CPRT reduced child behavioural problems and parenting related stress, while improving child self-concept. In contrast to other studies, all the children in Study 2 showed clinically significant improvement in their behaviour and this was maintained at a 3-month follow-up. Discussion includes limitations, implications for clinical practice and future research.