Background: Little information is available about prescribing patterns for patients with severe learning disabilities in Asian institutions. Materials and methods: Prescription audits were performed at Siu Lam Hospital (SLH) and Tuen Mun Hospital (TMH), Hong Kong, between 1988 and 2003. Results: About 65% of patients were receiving psychoactive medication. Male patients at TMH (but not at SLH) were more likely to receive psychoactive drugs than female (74% versus 58%). Polypharmacy occurred in around 30%. The proportions receiving anticonvulsants, lithium or benzodiazepines increased, while those receiving anticholinergics decreased. The most common indication for antipsychotic use was "acute psychomotor agitation" but 14-22% had no recorded indication. About 20-27% of prescriptions fell outside recommended daily dosages. Drug spending as a proportion of the total fell from 9% to 5%. Daily drug costs per patient were about HK$4. Conclusions: Changes in prescribing have occurred since 1988. Drug audits can improve prescribing for this vulnerable population.