Objective: To investigate whether changes in parenting after behavioural parent training in routine clinical care are associated with improvements in preschool children's disruptive behaviours. Method: We evaluated changes after parent training in maternal and paternal self-reports of parental discipline practices parenting sense of competence, and parents' ratings of child disruptive behaviours in parents of 63 children, with a one group pretest–posttest design. We also compared parenting parameters in this clinical sample with a nonclinical sample (n = 121). Results: Mothers' self-reports of parental discipline practices and parenting sense of competence significantly improved after behavioural parent training. Less over-reactivity in both mothers and fathers was associated with fewer disruptive behaviours in children. After parent training, mothers' ratings of their discipline techniques did not differ anymore from those in the nonclinical sample. Conclusion: Positive changes in parental discipline practices, particularly less over-reactive parental behaviours, were related to a decrease of disruptive child behaviours.