The present study describes the results of neuropsychiatric consultations in 70 institutionalized mentally retarded patients. On the basis of their major complaint, patients were assigned to eight overlapping categories. Diagnoses of a certain probability were established during multidisciplinary consensus meetings and classification was achieved according to ICD-10 criteria. In ten patients mental retardation was related to a specific syndrome; seizures or somatic comorbidity were present in 40 patients. The most frequent psychiatric diagnoses were: unspecified bipolar affective disorder (n = 14), depressive disorder (n = 6), impulse control disorder (n = 12), cycloid, transient, or schizoaffective psychotic disorder (n = 14), and (atypical) autism (n = 7). Recent history revealed a high occurrence of serious side effects of psychotropics or pharmacokinetic interactions. Appropriate pharmacological intervention resulted in an amelioration of the behavioural condition in about half of the patients. It is emphasized that psychiatric disorders frequently present with an atypical psychopathology and that stress- and anxiety-related disorders are most probably underdiagnosed.