Plasma fluphenazine concentrations (FLU) were measured in 45 patients with schizophrenic disorders who participated in a double-blind comparison of 5 and 25 mg fluphenazine decanoate (FD). The rise in plasma level of FLU 24 h after a "test dose" was significantly correlated with steady state FLU concentration at 12 weeks (for 5 mg patients, r = 0.45, P = 0.04; for 25 mg, r = 0.78, P = 0.005). Patients who had low FLU at baseline required nearly 6 months to reach a steady state when they received 25 mg. Patients who received 5 mg and had low FLU at baseline continued to demonstrate relatively low plasma levels for the entire 1st year. Although the mean FLU at 6 months was lower for patients who relapsed during the subsequent 18 months (0.57 ng/ml for relapsers vs 1.01 ng/ml for nonrelapsers), this difference was not statistically significant. When plasma levels from both dosage groups were combined, FLU at 12 weeks correlated significantly with factor scores for akinesia (r = 0.52, P = 0.002) and BPRS cluster scores for retardation (r = 0.52, P = 0.002). These results indicate that the measurement of fluphenazine plasma levels may be useful in determining when patients treated with FD are receiving drug doses which are likely to cause discomforting side effects.