A monoamine oxidase hypothesis for the cause of attention deficit hyperactivity disorder has recently been established (Shekim et al. 1986). To test this, moclobemide, a new and reversible inhibitor of monoamine oxidase-A, was administered to 12 children between the ages of 6 and 13 years, diagnosed as attention deficit hyperactive according to DSM III-R, in a 4-week study. All had previously discontinued treatment with methylphenidate due to side-effects. A 40% improvement in parental assessment of children's behaviour occurred among those who received moclobemide as outpatients, and a 31% improvement among the five in-patients. There was a general increase in concentration and attention span at the end of the trial, but mood changes and explosive behaviour continued to be present at a low level. Brain mapping after moclobemide treatment showed an overall reduction in frontal lobe delta activity. Moclobemide was well tolerated by these patients, although two complained of mild gastrointestinal disturbances during the first week of therapy.