Objective To investigate the effectiveness of behavioral parent training (BPT) for preschool children with disruptive behaviours and to explore parental predictors of response. Methods Parents of 68 preschool children, aged between 2.7 and 5.9 years, participated in BPT. We evaluated the changes in children's behaviour after BPT with a one group pretest–posttest design, using a waiting period for a double pretest. Outcome was based on parents' reports of the intensity and number of behaviour problems on the Eyberg Child Behavior Inventory. Predictor variables included parents' attention‐deficit/hyperactivity disorder symptoms, antisocial behaviours, and alcohol use, and maternal parenting self‐efficacy and disciplining. Results Mother‐reported child behaviour problems did not change in the waiting period but improved significantly after BPT (d = 0.63). High levels of alcohol use by fathers and low levels of maternal ineffective disciplining were each associated with somewhat worse outcome. Conclusions BPT under routine care conditions clearly improves disruptive behaviours in preschool children. Mothers who consider themselves as inadequate in disciplining and mothers whose partners do not consume high levels of alcohol report the largest improvements. Key practitioner Disruptive behaviours in referred preschool children improve when parents follow behavioural parent training under routine care conditions. Behavioural parent training is useful to help parents experience fewer behaviours as troublesome. About one third of the parents for whom behavioural parent training was indicated never started the treatment. Clinicians are recommended to put additional effort in motivating and facilitating parents to actually participate in behavioural parent training. It may be useful to assess and treat problematic alcohol use in fathers before behavioural parent training. Behavioural parent training may be particularly effective when mothers perceive themselves as inadequate in disciplining.