A minority of people with mental retardation have sexual behaviour which is socially unacceptable or which brings them into conflict with the law. Such behaviour may be the result of ignorance about sexual matters, often best managed by counselling or by a behavioural approach. There are a small number of men with mental retardation and aberrant sexual behaviour who benefit from the prescription of an antilibidinal drug. Treatment with an antilibidinal drug alone reduces the intensity of sexual drive but does not alter its direction; concurrent sexual counselling or psychotherapy is often indicated. The various antilibidinal medications available are reviewed, with particular attention to reports which have described the treatment of people with mental retardation. Of the antilibidinals currently used, medroxyprogesterone acetate and cyproterone acetate are those for which there is most evidence of efficacy. Cyproterone acetate is preferable because it has a specific antiandrogenic action and fewer adverse effects. There is a need for controlled studies of antilibidinal drugs, with clearly defined inclusion criteria and adequate measures of both behavioural and attitudinal change.