Cyclic changes in behaviour and mood which do not meet the criteria for bipolar affective disorder have been reported in people with intellectual disability (ID) since the beginning of the twentieth century. The present study postulates a functional disturbance of unstable mood disorder in such cases of an episodic pattern of disturbed behaviour, mood and anxiety. Since symptoms of hypomania or major depression are not observed in these individuals, the unstable mood disorder cannot be regarded as being part of the bipolar spectrum, although it resembles cyclothymia in some aspects. In this pilot study, 28 subjects with ID were treated with valproic acid in dosages leading to a mean plasma concentration of 63 mg L−1. A marked and sustained improvement was achieved in 68% of subjects in terms of both behaviour stability, and a reduction of symptoms in the mood, anxiety and motor domains.