The objective of this study is to compare the efficacy and cost of specialised individually delivered parent training (PT) for preschool children with attention-deficit/hyperactivity disorder (ADHD) against generic group-based PT and treatment as usual (TAU). This is a multi-centre three-arm, parallel group randomised controlled trial conducted in National Health Service Trusts. The participants included in this study were preschool children (33–54 months) fulfilling ADHD research diagnostic criteria. New Forest Parenting Programme (NFPP)—12-week individual, home-delivered ADHD PT programme; Incredible Years (IY)—12-week group-based, PT programme initially designed for children with behaviour problems were the interventions. Primary outcome—Parent ratings of child’s ADHD symptoms (Swanson, Nolan & Pelham Questionnaire—SNAP-IV). Secondary outcomes—teacher ratings (SNAP-IV) and direct observations of ADHD symptoms and parent/teacher ratings of conduct problems. NFPP, IY and TAU outcomes were measured at baseline (T1) and post treatment (T2). NFPP and IY outcomes only were measured 6 months post treatment (T3). Researchers, but not therapists or parents, were blind to treatment allocation. Analysis employed mixed effect regression models (multiple imputations). Intervention and other costs were estimated using standardized approaches. NFPP and IT did not differ on parent-rated SNAP-IV, ADHD combined symptoms [mean difference − 0.009 95% CI (− 0.191, 0.173), p = 0.921] or any other measure. Small, non-significant, benefits of NFPP over TAU were seen for parent-rated SNAP-IV, ADHD combined symptoms [− 0.189 95% CI (− 0.380, 0.003), p = 0.053]. NFPP significantly reduced parent-rated conduct problems compared to TAU across scales (p values < 0.05). No significant benefits of IY over TAU were seen for parent-rated SNAP, ADHD symptoms [− 0.16 95% CI (− 0.37, 0.04), p = 0.121] or parent-rated conduct problems (p > 0.05). The cost per family of providing NFPP in the trial was significantly lower than IY (£1591 versus £2103). Although, there were no differences between NFPA and IY with regards clinical effectiveness, individually delivered NFPP cost less. However, this difference may be reduced when implemented in routine clinical practice. Clinical decisions should take into account parental preferences between delivery approaches.