The behaviour dimensions of 244 Hungarian adolescent psychiatric outpatients with a dual diagnosis (intellectual disability and psychiatric diagnosis) were examined by means of the adapted version of the Behaviour Problem Inventory (BPI, Rojahn, Matson, Lott, Esbensen, & Smalls, 2001). Four IQ subgroups were created: borderline, mild, moderate and profound ID subsamples. Significantly higher means were found in the self-injury/stereotyped behaviour/summarized scale categories both in the frequency and severity of symptoms in the more disabled groups against the samples having milder IQ impairment. Adolescents with a dual diagnosis showed much higher BPI scale means than an adult residential ID sample. ADHD and emotional disorders were the most frequent psychiatric diagnostic comorbidities of ID (20.67% and 11.73%). Academic achievement disorder, depression and psychosis had low occurrences (3.35, 2.23 and 1.17%, respectively) but showed convergency with other authors' data. The comorbid emotional disorders may create challenges for the care of the mildly intellectually disabled group. (Contains 3 tables.)