Low doses of neuroleptics are the standard for treating psychosis in elderly patients because of concern about inducing adverse effects. The authors found that fixed, low-dose neuroleptic treatment (0.15 mg/kg/day) for 10 days resulted in low perphenazine levels and low rates of acute response (25%) in elderly patients with primary psychotic illness (without dementia). Increase in initial dose did not speed acute response and induced adverse effects that were absent or minimal with low-dose treatment. With higher-dose treatment, drug blood levels rose disproportionately, and level-to-dose ratios were higher than those observed in non-elderly adults. Naturalistic follow-up suggested that response may take longer to develop than in non-elderly adults and that low doses for a longer duration may provide effective treatment.