We carried out an 8-month double-blind clinical trial comparing haloperidol decanoate with fluphenazine decanoate in the maintenance treatment of 72 schizophrenic outpatients. A parallel-groups design was used with stratification by sex and injection interval (2, 3, or 4 weeks). The initial injection interval was based on pretrial maintenance treatment with fluphenazine esters. The dosage equivalency of haloperidol decanoate (1.5 cc or 75 mg) to fluphenazine decanoate (1 cc or 25 mg) used was 3:1. This remained approximately the same throughout a 2-month titration period with a flexible dose regimen, and a further 6-month period with a fixed dose regimen. No statistically significant differences in therapeutic effect were found between the drugs. Both drugs had a similar profile for drug-induced parkinsonism, but there was a trend for differences in masking tardive dyskinesia. Haloperidol and prolactin plasma concentrations were well correlated with dosage, with the exception of haloperidol concentrations in patients receiving injections at 2-week intervals.