To identify the distribution of CYP2D6 metaboliser status in patients who were being prescribed risperidone for the treatment of psychosis in a New Zealand-based clinical population.100 AmpliChip CYP450 Test kits were made available by Roche Diagnostics. Clinicians in mental health services across three Auckland District Health Boards were instructed that the tests were being made available for use with patients who were being prescribed risperidone for the first time. Test results were fed back to the prescribing clinician. Data analysis was descriptive in nature; however, chi-square and independent sample t tests were employed to examine differences in age, gender, and ethnicity.Data were obtained for 93 patients. Poor and intermediate metabolisers each constituted 10.6% of the sample. There were no ultra-rapid metabolisers. Statistical analysis revealed no significant between-group differences with respect to age or gender. The between-group difference in ethnicity status showed a trend towards statistical significance.Sample size limitations likely contributed to the finding that no statistically significant between-group differences were identified. In theory, though, for one in five patients a higher level of adverse effects might be predicted for a normal dose of risperidone, potentially leading to issues around treatment adherence or treatment failure.