This study examined the effectiveness of the Parents Plus Programme for families of preschool children with significant behavioural problems, comparing those with and without developmental disabilities. Twenty-two parents of children with developmental disabilities and conduct problems (the disability group), and 17 parents of children with conduct problems, but without developmental disabilities (the conduct problems group), were assessed before and after participating in the Parents Plus Programme, and at 10 months follow-up. More than 70% of cases in both the disability and conduct problems groups showed clinically significant improvement on the Total Difficulties scale of the Strengths and Difficulties Questionnaire. During the follow-up period, parents in the disability group showed a deterioration in psychological adjustment (on the General Health Questionnaire-12), while parents in the conduct problems group showed an improvement. Parents in the disability group reported a higher level of goal attainment compared with parents in the conduct problem group. Parents in both the disability and conduct problems groups evaluated the Parents Plus Programme equally positively. The Parents Plus Programme requires refinement to become more effective for families of pre-school children with developmental disabilities.